Why aren't all research articles on PubMed?
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.
from here
The National Library of Medicine uses an NIH-chartered committee, the Literature Selection Technical Review Committee (LSTRC), to review all new biomedical and health journal titles and recommend those to be indexed for MEDLINE®. The NLM Fact Sheet titled "MEDLINE Journal Selection" provides information on the role of the LSTRC and critical factors considered in making the decision to recommend a journal title for indexing. It is intended only as a guide since there is no one set of characteristics that will guarantee selection. The LSTRC considers the quality of the scientific content, including originality, and the importance of the content for the MEDLINE audience throughout the world.
Often there are journals that clearly have bio-medical content, and are not found in PubMed. That's because registering a journal in PubMed is a non-trivial task. Even for big players like Cell Press it took almost a year to put "Cell Reports" in there.
The publisher can apply to be included in PubMed, but that usually takes some time. The meeting where it is discussed if a journal is included is only held twice per year. One requirement is that the journal is also available in print. That's why many journals start with a print issue and then after inclusion in PubMed drop it again.