You tend to need to add features in unforeseen ways. Popular large frameworks can facilitate this more than micro-frameworks can, resulting in less work on the developer's part.

micro frameworks provide incomplete feature set. You tend to reinvent the wheel many times. I see no advantage in using them when I can have something more complete and feature rich.

Because as your app grows and you need more custom behavior, you find that you start stripping out the full-stack framework features and you end up with using it like a micro framework, which it wasn't designed to be so you have now have a framework mismatch.

pylons is a glue framework. It allows you to replace whatever you want, easily and without losing other framework features.