Why do some fields not use arXiv? My field is theoretical cosmology. I use the arXiv to find virtually every paper I read. Most preprints are updated with their peer-reviewed, published versions so I don't have concerns about their scientific legitimacy. However, when I've had to work on assignments or projects in other fields or subfields for courses at university I find myself constantly running into journal paywalls, which is very frustrating. For this reason I want to know why the arXiv's system of free access preprints is not more widely used.

In this answer it is stated that


  Compared with other fields, mathematics and physics are anomalous in making heavy use of the arXiv, with physics even more so than mathematics.


This tallies with my experience. The answer then goes on to say


  Things are trickier in other fields, because many of the advantages depend on network effects. If nobody in your field pays attention to a server, it's not conventional, and it counts for little or nothing regarding priority, then there's less reason to post to it (although it still has some value).


Is this the only reason that some fields don't post preprints on arXiv? Should we expect to see more fields publishing on arXiv in the future as the word gets around?
There are several reasons why a field might not use arXiv, or another pre-print service. Whether or not these are correct is left as an exercise to the reader:


It will interfere with publication. This is a little circular, but if a field doesn't use pre-prints, journals might not recognize a pre-print being put in a depository as not prior publication. Whatever merits there are in putting something in a pre-print repository are offset if you can't get it into a journal.
Pre-prints create version sprawl. If there's significant revision of pre-prints, there's now sort of two versions hanging out, the one on the pre-print server and the one that sees publication.
Priority is not necessarily as important (not that it isn't important). In biomedicine, the ideal is for multiple studies to all independently estimate an effect, so being first isn't as critical.
Pre-prints are not peer reviewed, but a pre-print of an RCT or something may still be used to influence medical decision-making etc.
Pre-prints and double-blind peer review are basically mutually exclusive. This one is, in my mind, the biggest downside of pre-prints.


When it comes down to it though, much of it is just field convention - if preprints aren't used, there's very little utility to being a "pioneer" in that regard - it's work, and explaining, and some risk for very little benefit.