Q: Why is certain literature not available on PubMed, but is available on PubMed Central (PMC)?
There is [some] literature that is not searchable/available on PubMed, but is searchable/available on PubMed Central (PMC). Why does this happen? Is there any reason for this?
Please note that we have made some edits to your question for enhanced comprehension, and have responded accordingly. We hope our understanding of the question, and therefore, our response, is correct.
So, yes, there are some articles that you can find through PMC but not through PubMed, just as there are some articles you can find through PubMed that you can’t find through PMC. As PubMed is the broader database (including articles from beyond PMC), this should logically not be the case. However, this is because (and without getting too technical) of the way the two databases have been set up, or rather, the reasons behind their existence.
PubMed provides only abstracts and links to complete articles, which may or may not be free. On PMC, articles appear in their complete form and are all free. PubMed is meant to provide you a greater range of papers, as it lists papers from several sources. PMC is meant to provide you a greater depth of papers. So, PubMed is great for doing some initial, high-level research, in order to get an idea of the current research in a field, especially as it also lists papers from beyond biomedicine and life sciences. When you wish to go a bit deeper into a topic, especially from biomedicine and life sciences, PMC would probably be more useful. For best results though, it would of course be best to look up both databases.
Hope that helps. For more on PubMed searches, here are some previous queries by other researchers that you may find useful.