Because I think Pac at 145 or whatever is a bigger scalp then JMM. Mosley would have wins over the former so-called best welter in the world and the hottest guy in boxing. Not like it matters. The winner is gonna fight the winner of the JMM Mayweather fight anyway.
No one gives a **** if he's undefeated. Beating JMM is not as good as beating Pac. Especially as JMM hasn't fought above lightweight before. Beating Margarito at welter is better than doing nothing for two years.