I want to know why some PhD students don't publish their works in a journal.
For example, I have seen many good, high level German theses which were written around 1970-90, but were not published in any journal.
Academia Stack Exchange is a question and answer site for academics and those enrolled in higher education. It only takes a minute to sign up.Sign up to join this community
The obligatory journal publication of scientific work done for a degree, no matter what (think: "publish or perish") is a fashion that spilled over from the US to Europe and especially Germany around the late '80s-'90s. Before that, people bothered only publishing work that they felt was outstanding, and sometimes not even that.
I am aware of work that exceeded in thoroughness and quality by far other publications on the same question that were published more than a decade and half later, where the original authors did not feel that it was up-to-scratch for publication and refused to do what they felt was a "half-baked" work.
One may say it might be desirable if some this self-critical attitude would still be maintained (not in its totality, so much good work went effectively lost this way), it would save one a lot of sub-par reading and reviewing duties today. However, this ship has sailed long ago.
First, in the olden times there was much less pressure to publish, and many researchers would publish only a handful of papers in their whole life, and they would publish only very complete works. I personally know researchers, considered anyway leading researchers in their field, who would publish just once every few years and who retired with probably less than ten published papers. Something that nowadays wouldn't even get you a tenured position.
Second, many PhD students leave academia for industry once graduated, and they may lose interest in publishing their work, right at the moment in which their work is more mature for publication.
There are good answers pointing out things were different in the '80s and '90s. Another thing to keep in mind is the differences between disciplines. There are disciplines, e.g. (some parts of) history in Germany, where journals don't have the standing it has in other disciplines. Slightly exaggerated (but only slightly) the attitude is that the only really scientific publication is a book.
There are good reasons for that: history, by the very nature of the thing they study, is not a very fast paced field. So if the speed of journal article writing is not a big benefit, then why not use the space of a book to make a really thoughtful argument? In those disciplines publishing your thesis in journals is still not encouraged, and in part actively discouraged.
Surprisingly not yet mentioned in the comments or in the 4 answers thus far is that for some people, and this is mostly for the 1970s to early 1980s, there was also the problem of typesetting journal papers for submission if you were no longer in an environment with department secretaries to type your work (because writing papers is not "mission central" at non-research colleges/universities, such as The University of Southern North Dakota at Hoople). The situation was even worse if you were not in an academic position or if you were in a non-academic research oriented position that was not closely related to whatever esoteric subtopic your dissertation dealt with. Obviously this was especially problematic for someone who never learned to type (not all that uncommon before the early to mid 1980s), but even for those with competent typing skills, the task of preparing multiple copies (via carbon copies) of a nearly error-free document, full of math or other technical symbols carefully handwritten on the copies, often just wasn't a sufficiently high priority for the effort needed, especially in those cases where there would be essentially no career or other advantages in doing so.
I can think of a few reasons specific to the period you describe:
Most of the other answers focus on the theses written prior to 1990 not being published in journals. However, even nowadays theses are sometimes not published in journals for a variety of reasons – including mine from 2017.
This will, depend on the field, obviously; but my thesis was written in organic chemistry which is traditionally a field where journals are valued highly. My specific sub-field (total synthesis of natural products) has a couple of difficulties that, taken together, prevented my work and the work of most of my colleagues during their PhD from being published.
Publications in this sub-field typically consist of a complete synthesis of a large and complex molecule; one entire PhD typically consists of synthesising only one such compound. (In some cases, maitotoxin being the extreme, multiple PhD students work on the same compound over the course of many years, sometimes even in succession.) At the end of one’s time, there are two possible outcomes: a completed synthesis which has a high chance of ending up in a top-tier journal or an incomplete synthesis which, at best, can be published in a very low-tier journal – even if the work itself is thorough, high-level and extensive.
Unfortunately, it is not really possible to predict whether a project will be successful and what time it may take. My project was initially given to a master’s student because it seemed easy to complete and when I took over, I expected to complete it in a year maybe a year and a half and move on to a second project. This did not work out and ultimately another PhD student took over when I was finished – but she also was unable to complete the molecule. In the end, we settled for a partial synthesis in a lesser journal, probably because my supervisor was completely fed up with the compound. This partial synthesis covers maybe 40 % of what I included in my thesis (the PhD student who took over declined authorship, stating that she had provided no significant improvements or additions to my data).
On the other hand, one of my colleagues had a similarly complex molecule but a couple of strokes of luck leading to his completed synthesis being published within his first year as PhD. He spent the remainder of his time synthesising related natural products and derivatives and ended up with three papers to his name. (This being natural product chemistry, there were three people listed as authors: the PhD student and his two supervisors.)
The above leads me to another aspect: publication in a journal is often not mandatory to receive one’s PhD. At my university, publications were entirely optional. On the other hand, instead of writing a separate thesis three (or more) publications of similar or related topics could be submitted together with an introduction and a conclusion in what is known as a cumulative thesis. My colleague who ended up with three papers chose this path.
In other sub-fields of organic chemistry, publications are far more predictable. However, in these fields a PhD thesis often encompasses several individual projects that may or may not be strongly related. Individually, these projects are sufficient for publication, but it might not be possible to publish the entire thesis in one, as the projects aren’t related sufficiently. So the entirety of a thesis may be split across several publications rather than collected into one.
Well, talking about the period the OP mentions (70s-90s) the 'publish or perish' idea wasn't really a thing. So then chances of publication would depend very much on if the supervisor of the graduated doctoral student had much interest in said students work. Coupled with the fact that staying in touch, co writing drafts and such was harder pre internet. Both of my parents have PhDs (UK, early 1980s) from which nothing was ever published in a journal. My Dad was explicitly told by his supervisor when starting his research 'If you publish, it won't be with me.' (an attitude that mystifies me, but it was the case). Bits of my own thesis (Germany 2010s) remain unpublished as my supervisor was less interested in those particular bits and / or they needed just that bit more work, topped off by the fact he retired a few years after I finished.
So really in summary, there are a whole pile of factors, the ones covered here are just a few examples.
While I don't know about German universities in the 1970s-90s, there is (at least in my experience) a fairly obvious reason why PhD dissertations (and master's theses) aren't published in journals: they're way too long.
The typical journal paper runs about 8-10 pages (per Google and my own experience), while a thesis/dissertation might be closer to 100 pages. Many journals impose page charges - $100-$200 per page - so publishing papers of that length in traditional journals would be financially impossible for many people. https://www.nature.com/news/open-access-the-true-cost-of-science-publishing-1.12676
What often happens (again, in my experience) is just the opposite: the research will be published in several short papers, which will be expanded to form chapters of the final thesis/dissertation. Then too, if the research is part of a larger project, the papers might well be published with several co-authors.