We do mention BitBucket in the [forking workflows section](http://git-scm.com/book/en/v2/Distributed-Git-Contributing-t...), and many of the lessons from the GitHub chapter will carry over; the two sites have a lot in common.
The conflict of interest does bother me, so I did try to be as clear as I could about it and to point out at the beginning of the chapter that you could easily skip it if you don't like or don't want to use GitHub. I also stayed away from everything I could that required payment.
The truth is that I've been approached to write books specifically about GitHub for several years so I thought that the demand for that sort of information was high enough to warrant moving it to it's own chapter. Had I been approached about writing about other resources I would probably have also included them.
The more difficult decision was whether to include information on Gerrit. I actually started writing a section on it, then considered a chapter or appendix. The main deciding factor was how many people I thought might benefit from it.