A loss agsinst Mayweather would have a much worse effect than that KO by JMM or getting robbed against Bradley; he's still Pacquiao.

With Mayweather, it's his nemesis we're talking about and perhaps Arum doesn't want the fight to happen for that reason: it would burst what's left of the bubble, meaning less money for Arum in future fights, giving Floyd even more star-power, etc.