The Marquez fight was the last time I saw Floyd using his legs and picking spots to fight. I saw flashes of it against cotto, but mostly walking people down and fighting in front with superior defense... Why? Is it being physically stronger? Slower reflexes? Slower legs? Less stamina over course of fight? Looking for ko's? What? I remember an interview after Mosley saying "I knew I could fight like this"
It could be a number of things. I think its a combination of wanting to prove to everyone that he doesnt have to use his legs so much and move around the ring and also he wanted to prove to himself that he can fight with any style and still dominate.
He's getting older, his legs are going, and he's adapting his fighting style to compensate his legs. Because his inside game is so good, and his defense is still so sharp, he's able to transition into this different style. Also, he's been at 147 for some time now so he's really growing into the weight. Trust me, it may not seem like it, but he's packing power nowadays.
Mayweather's legs are fine. He's obviously not going to be bouncing around the ring like he was when he was in his early 20's, but none the less, his legs are fine. Look at all his fights since that second Castillo fight. The second half of the Baldo fight is the only one where he used a lot of movement. He's 36, so he has slowed down. However, his legs are still plenty good enough.
He doesn't have the explosivenss and spring in his legs anymore. Happens to every single human being right around 35. He's simply adjustnng his style to maximize what he has left, which is a credit to his adaptability and ring smarts. I think if he could, he would fight the way he did against Marquez but his reflexes and other physical attributes just aren't there.
Had Mayweather fought Marquez the way he fought Cotto and Mosley, he'd have been in a real fight. Speed, quickness, combined with his height and reach is what you do to beat a blown up lightweight who's boxing skills are on par or surpass yours! Get a clue clowns? :deal
Age and size. I'm sure if he could rely on his foot defense and movement to make people miss more often than not, he would, but he more than likely prefers strong legs over the course of 12 rounds rather than employing a lot of movement and his legs giving in half way through. Saying that, the Cotto bout was the first time (one of the very rare times) I've seen Mayweather feeling the pace of a bout.