An article on The Chronicle of Higher Education called: The Road From Dissertation to Book Has a New Pothole: the Internet raises some important points about whether a thesis that exists online can still be published.
The important thing to consider is that most publishing has to function as a viable business model. If a thesis has been freely available online, why would anyone buy it? And you need to bear in mind that when your book is sold, it’s likely to be funding not it’s own production, but that of the next book in line, so you’ve got more weight on your shoulders than just your career.
That said, as Gary Hall elegantly argues in Digitize This Book, these business models are being rapidly redeveloped by forms of online content sharing. So for example, your work might reach a bigger audience by being freely available online and this might become a surer route to career success.
Also, remember that presses seldom publish a thesis as is. We have a series coming up by Sarah Caro, author of How to Publish Your PhD, that explains how much redrafting must go on before your average thesis is even remotely book-shaped.
Or you can take my route and work on a book that expands one element of your thesis and maybe come back to tackling the whole thing later (losing that element if need be). Or just have done with your thesis altogether, see it as a ladder you climbed to get this far and then kick it away and start on the next one…