Why do you think Manny Pacquiao never went back down to 140?

Bingo.

The Original 8 divisions are historically more rich and carry more prestige (and, as a result, usually present more money) as a result. They're "endgame" divisions. If you can make it in one, you don't move up to a junior/super divison unless:

a) you simply can't make the weight anymore (example: Arthur Abraham)
b) it's on the way to another of the original weight classes (example: Oscar at 154 before going to middleweight)
c) it's a one-off kind of fight (example: Manny fighting Margo, Canelo vs Fielding)

Dudes that would be better served staying at 140 routinely make the move up to 147 for the reasons above & what you mentioned.

 

Was no money to be made.

 

True but but I think it was still a special performance. It looked like Hatton thought he could bully Pac. He was wrong and it was a spectacular KO.