The original question was.... why don't fighters clean out divisions before moving up?

They do not clean out their divisions, or do not have to, if they have the ability to pursue a bigger game, a greater exploit in higher weight classes. Likewise, if the move will serve to be a financially rewarding endeavor and is a potential to add feathers on their caps.

The fighters who are cleaning out their divisions are mostly the one's that are willing to do so for their convenience. It is mostly but not exclusive to, their inability talent wise, or their lack of confidence for such a move anywhere north or south of their best weight class.