A thesis is an examination document and a unit of work which must be produced in order to be awarded a degree. There is some evidence that this does not affect publication in the Sciences where the unit of publication is much shorter (focused journal articles, for example). For book publication, particularly in the Social Sciences and Humanities there is more concern.
A published version of a thesis is usually aimed at a different audience, peer reviewed, re-written, significantly edited and presented more attractively. Some publishers raise concerns about prior disclosure of research findings and it may be necessary to embargo access to a specific thesis, which is catered for in the depositing of theses with the University. It is something you need to take into account if you are intending to rewrite your thesis as a book or you may wish to obtain agreement with a Publisher earlier on in the process about releasing the original thesis giving some thought to the Copyright issues.