Why are dissertation-based books looked down upon? I came across the following statement by the Chicago University Press:


  If your book has its origins in a dissertation, your acknowledgments should not draw attention to this fact, as it will discourage library sales and book review attention.


Why are dissertation-based books looked down upon?

A dissertation-based book is not necessarily of low quality. I was surprised to read such a statement from a respected publisher. Isn't this unprofessional? If there is such a stereotype, shouldn't a publisher like the Chicago University Press refrain from nurturing it?
Simply compare the number of dissertations and books published in a field. A dissertation can be strong enough to provide enough material for a technical book, but it is rarely the case. A dissertation is normally focused on a specific research work, but a book is expected to be comprehensive. In rare cases, a PhD student has a comprehensive perspective of the field to write a flawless book. 

It is legitimate and understandable that a publisher (university-based or independent) pays a particular attention to the marketing factors. The readers normally expect a book written by an experienced academic rather than a PhD student.

The publisher simply avoids highlighting any point, which might have a negative impact on the potential buyers, and leave them to judge the book based on its contents. They simply want to avoid prejudice about the book.

So, don't worry, they do not undermine the quality of PhD dissertations.

And believe me, marketing is as technical as your field of study, and has its own rules.