Well, besides the obvious fact that Vegas odds makers are far from infallible and odds don't really accurately represent the dangers of fighting an elite level boxer, I think the odds difference can be explained largely by the fact that Canelo was the much bigger man who Floyd was moving up to fight, and was also undefeated.

Maidana is a rough customer and he's not going to quit or beat himself in there. He's going to come out firing and make Mayweather work. This is not a "gimme" fight. The odds would have been equally bad (or at least close to it) if it was Khan fighting him as well. It's just a testament to how dominant Floyd is in the sport.