You could also flip the question around as well in a lot of cases..

Why don't fighters try moving up a division when they have completely cleaned out the existing one..

Hatton is a good example of someone who did... He really had 140 owned and was absolutely no doubt the world champion at that weight.. And moved up which I think was a brave thing by him...

But I think as Mick said about Pac, as long as you at least take on someone in the division with credit, and beat them in a manner where people can really reasonable argue that you'd be the favorite against anyone else in that division, then moving up is okay...
And also when you think about it, moving up is the bigger risk and challenge anyway... Not in all cases... You can find people at a lower weight who you'd pick over a lesser ranked fighter at a higher weight..