Anecdotal, but in my org's case, vendors providing little to no documentation seems to be due to one (or more) of a few things:

1. Ensuring an ongoing support contract that wouldn't be needed if the product were properly documented (this is in addition to the software maintenance contract for updates/patches).
2. Nudging us toward their hosted, SaaS solution (and significantly higher ongoing costs)
3. They're just generally terrible in one or more ways: writing documentation, disorganized development and project management practices, etc.

My org is a non-profit that deals with environmental remediation and often has to electronically submit data to the US EPA to qualify for grants, so pretty much all of the software we use is niche; more often than not, the lowest bidder is also the only bidder and we have to take what we can get. :sigh: