Top Rank simply recognizes that Cotto is coming off a dynamic performance and has a huge, loyal fan base. He'd also be fighting for history against Sergio so the PPV has success written all over it.
Cotto-Canelo would definitely be a bigger fight, but Canelo at this point is much more expensive than Martinez to get in the ring. Martinez "only" got 1.4 mil to fight Chavez Jr, so I'm assuming that Bob thinks he'll fight Cotto for a relatively low purse, and bottom line I think he's just trying to keep Cotto from going to his hated rivals, GBP.
I'm not sure the PPV will do all that well given that Sergio has a relatively small fanbase, but Bob knows Cotto is still very marketable and the last thing he wants is for GBP to snatch him up and make all kinds of money off of him.
i don't think gbp will pay cotto 10mil against canelo or that tr is going to pay cotto 12mil to fight sergio.
GGG is suicide for cotto sm and canelo.. but cotto has a real shot at sm, especially if its sm next fight. canelo KOs cotto every time. too big and strong and those power combos will land on cotto and take him out. canelo also stops sm every time. 2 good fights for the fans though. i feel for GGG, nobody will fight him for good reason, biggest beast in boxing..