Sinatra is cool. However, then I need to start thinking about how to integrate database connectivity (configure it, set up controllers, what not). I need to worry about XSS attacks, and a zillion other tiny details that go into a web app. So I often end up choosing Rails anyway, because they made a lot of choices for me already that I frankly don't care much about making myself.

Still, Sinatra is great for a lot of small web projects.