What are the main factors that mean Humanities PhDs get published as books?

In both this comment and this answer it is conveyed that in the humanities, PhD thesis are viewed as a first draft of a book and are often published. There are probably a number of factors that have led to this view.

I'm interested in a number of issues. First of all, is there hard data that shows how many PhDs by discipline get published as a book?

I have no knowledge of the publishing process but it would be interesting to know in this case where the driving factors are. Does the student themselves seek to publish or does their supervisor/committee/department inform the student that they think their material could be a book. Do the publishing companies actively seek material to publish or expect the author to approach them. Are there other factors unique to the Humanities that mean the book is a preferred publishing route?

So to round up, what are the main factors that mean Humanities PhDs appear to often get published as book?