Because he would not outfight Castillo inside. And I don't think he could throw the combos or move the way he would need to if someone comparable to Castillo stepped in the ring with him today.
I'm not guaranteeing Castillo would beat this Mayweather, but I know he would do better than in the rematch, which people overblow as if Mayweather whitewashed him.
You might have forgotten the 11th and 12th round of that fight, that fight proved that Mayweather could have stood in the pocket had he wanted to, but Floyd felt that boxing Castillo would have been the easier approach (That was proven in the rematch). Jump to 3:35,