I would describe it more carefully. Bottle's advantage that it's self contained and in a single file, however bottle's design also hides away the application object from you which makes it quite hard to have more than one of them. It is possible with the stacked object thing and I think they changed it even to make it possible to do what Flask did, but when Flask started at least there was a design difference there.

By now I really would like to see them merge, but Bottle wants to stick to the design idea of just having one file which Flask will never be able to do because of the dependencies.