It's very simple, aside from what the two previous posters said, Guerrero is a GBP fighter=easy to make the fight. Guerrero is coming off the best string of wins from fighters Mayweather could actually fight. Guerrero is Mexican=May 4th.
Marquez is not going to fight Mayweather again. You know damn well Floyd would sign the dotted line for an easy payday against Marquez. Like the poster above said, Marquez showed his age in the Pacquiao. Yeah Pacquiao's defense once again failed him, but Marquez showed his age. Just like I said, this brand new muscular JMM would be the worst of all 4 fights, and technically he was. He was slow, was losing by the most rounds ever from all previous bouts with Pacquiao. A well placed punch got him out of trouble for good, he should retire now.
I'd love the Canelo fight right now, but I agree that if Canelo gets a good win on May 4th, then the Mayweather fight would be much better in September.
Martinez is fighting in April.