Because his chances are not the same anymore just like his power, speed and defense maneuvers. He cannot hide that fact that he is already old even if he's still fight in exhibitions, he will have the ring rust effect if he will choose to deal with the likes of Pacquiao nowadays, a May-Pac fight is profitable but knowing Mayweather, he doesn't want to get embarrassed from being defeated.
On the contrary, I think Mayweather will still win if ever May-Pac fight, exhibition or not, happens. We may see Mayweather's skill deteriorates but the same goes with Manny. With the defensive stance of Mayweather, I believe Manny needs more energy, and quicker reflex to break Mayweather defense while Mayweather can just do what he normally do.
Why no one discussing (or havent made a topic) about Deontay Wilder vs Robert Helenius fight ? Only 3 days left. Is it because Tyson Fury has destroyed Wilder reputation after beating him twice, and people have completely forgot about this knockout artist ? There were so many knockout compilations on YouTube, and now, in two years, he is forgotten. Helenius is not a biggest boxing superstar, but 31-3 record should be taken into consideration. He is not a trashy punching bag?
We can blame that on the promoters. Besides, there is not much of attraction there, Wilder's reputation was greatly damaged when he was beaten by Tyson Fury, and his current opponent isn't sensational. So with lack of hype, only a few got interested on this fight.