Because Bootstrap's typography and layout (line heights, margins and spacing, etc.) is way better than most everything else, including browser default stylesheets and most WP themes.
Because if you do want to customize it further, there are an increasing number of people who find it most efficient and powerful to do that on top of Bootstrap, and a lot of code samples/templates for doing so.
I know that the official line is (or was?) that Bootstrap is 'a starter place, for wireframe prototypes, not for mature projects' -- but that is not in fact how it's been used. I believe the reason it's been so popular is because of it's excellent typography, as well as fairly well-structured easy to work with code.