Every framework has its own set of pros and cons. Sinatra is not the perfect framework for everything. Here are some of them.
Why Sinatra?
You want to be flexible and choose your own gems and libraries. Unlike Rails, Sinatra lets the user choose whatever gems s/he wants to use.
You want a higher performance. Sinatra is lightweight and does only the stuff that you want it to do.
You want to make a completely modular application.
You want to build an API.
You want to learn Rails.
You want to implement concurrency.
Why not Sinatra?