A shorter book is more attractive on several levels – booksellers prefer shorter books because you can fit more of them on a shelf. A large book by an unknown author could be taking up room which could be used by someone else. Someone more famous, for instance.
Readers prefer shorter books. Your book is more likely to appeal to more readers if it is short. If you are an unknown author, readers want to take less of a risk, and having fewer pages to read makes your book less risky.
Finally, for the self-published writer the incentive is financial. If you’re sending out a book via Royal Mail (for example), you can save a lot of money sending out books that are classified as large letters, rather than small parcels. At the time of writing, a large letter costs £1.68 to post (UK only), whereas a small parcel costs £2.80.
The large letter classification is under 25mm thick. In practice, this means anything of 300 pages or fewer will qualify – and that extra £1.12 is purely the author’s saving.
For a guide to word counts for books, see this Writers Digest post.