In my experience it tends to be challenging to actually find all the packages
needed for that. And then it's incredibly slow -- seems to be much slower
than gcc which is somewhat of an archivement. And at least for LinuxDoc TeX usually can't even compile the result.
I would say the track record of existing DocBook deployment is not good enough
to justify further use. Plain html can be converted into all these
formats easily too and overall it makes a much nicer user experience.
Also I would expect much more people will know how to write html versus
DocBook.