This came up just the other day in a class - journal vs conference publications.
Background; pros and cons:
I came to CS from neuroscience, a field (like most?) in which journals were the primary currency and some of the larger conferences accepted posters / talks but with only a minor vetting process. Conferences were often used to present preliminary work and get feedback on methodology, journal articles were for the meat of the work. I had many conference posters (and one or two journal papers) on my CV when i applied to CS programs and was surprised at how professors were impressed by that - it seemed pretty common for people in neuro to have a good number of conference posters/talks.
Conference papers have low latency of acceptance/presentation. Typically deadlines are 4-7 months out from the conference, which tends to be quite a bit faster than journal article acceptances / publications. In my relatively short career, i've worked on papers (in neuroscience) that took 2+ years to publish, from intial submission to revision rounds to acceptance and publication. Conferences also tend to give your work more visibility - I recently presented my recent research at an SC15 workshop and had more feedback and potential collaborations come out of it than i ever did from my 1st-author journal article or from poster sessions in neuro conferences.
Journal articles allow for considerably greater depth and treatment of the topic at hand and are much more useful for reproducibility and understanding. Conference papers tend to have hard page limits and if the work is substantial enough, you can be forced to gloss over detail and have the potential to not ever publish follow up papers to explain the missing details.
A good issue around this, and this is what we discussed in class, was Google's Spanner paper, presented at OSDI. The paper is groundbreaking and incredibly impressive work, but due to the limitations of the format (short paper, again), much of the detail and nuance needed to understand the work is left out. The paper even mentions that a follow up paper would be forthcoming - three years later and it hasn't materialized. For such a huge piece of work that encompassed years of principle engineer time at an org like Google, it was probably better suited to a 30~ page journal article, but it may not have had the same impact as it did being presented at OSDI - one of the premier systems conferences.
To answer the question at hand, (why is it like this), many people have written about it (at conferences and journals!). One longer discussion can be found here.
Perhaps it is as simple as "this is how it has been for a long time and change is hard" - not particularly satisfying, but might explain a great deal of the variance here.