Because PubMed is about biomedical literature and life science journals. Things are blurry, however, because of two factors:
- Obviously, there is no strict definition of what is (or is not) biomedicine.
- PubMed does not select “article by article”, but has a list of indexed journals, from which all papers are indexed. Yet, a single journal's scope can be very varied.
Combine the two, and you get the idea that there is a lot of variability in whether an article on a given topic is published. For example, my own research field is materials chemistry, and about two-thirds of my papers appear to be indexed in PubMed: probably because those were published in generic chemistry journals, which also happen to publish some papers of relevant to life science.