There are probably lots of reasons, but I'd imagine one of them is that a simple Sinatra app consumes less resources than a simple Rails 3.1 app, and if Heroku is going to be adding tens of thousands of them, the savings probably adds up.
Each dyno (isolated running process) is given the same type of container to run in and the difference in data storage is trivial (a couple megs of gems).
In this case it's more about making it simple to get started. Rails has a fairly large learning curve, and the asset pipeline in 3.1 has only made it worse.
