Dumb thread, TBH.
When Mayweather-Tszyu was at it's best moment to happen, which even Floyd called him out on HBO earlier that year, Tszyu and Mayweather fought in the same exact month vs Hatton and Gatti respectively. Mayweather dominated and destroyed Gatti, Hatton dominated and retired Tszyu. Hatton was not ready for Mayweather however.
Margarito was a TR fighter, Floyd had just left TR chasing for a DLH fight in 2006 because DLH said he will not work with Arum. DLH-Mayweather did not happen in 2006 as it was thought, because DLH wanted to get a belt so he fought Mayorga. In the meantime, Floyd fought WW lineal/Ring champion Carlos Baldomir.
Pacquiao had the chance to make the fight in 2010 but backed away from a 15 day cut off, which he himself had asked for. It was granted to him by Mayweather but he still backed out. Just like he told Martinez to come down to 150, Martinez said okay, but Pacquiao backed out.