What are the most important points to emphasize while pitching for a dissertation to be published as a book?
Editors need books that will have an audience, a point, impact, and a connection to their "lists". 

In a conversation you should be able to briefly summarize your manuscript -- and in this summary make clear its central argument / narrative, scope / evidence base, and its what is novel, important, and exciting about it. You should also summarize the audience for the manuscript,  -- how it fits into the market: what other books are competing for this audience, and what distinguishes your manuscript. These summaries should be honest, and direct -- avoid both negativity and overstatement. Choose language to appeal to a smart, and critical, but not  specialize reader -- avoid jargon, and if you have to use specialized terms, define them briefly, in common-sense terms. 

For a written prospectus, follow the same model, but expand to two pages; and include an outline of the work, list of figures/tables, and note any special features such as data, or an accompanying website. 

For more details, see: Germano, From Dissertation to Book and Luey, Handbook for Academic Authors