Money I believe. Floyd wanted 50-50 because he was number 1 P4P and he already beat Oscar. I think the fight was going to be at 147 for Floyd's WBC title.
Regarding the first fight: Judge Chuck Giampa scored it 116-112 and Jerry Roth had it 115-113, both for Mayweather, while judge Tommy Kaczmarek had it 115-113 for the Golden Boy. ESPN.com had it 116-112 for Mayweather (38-0). Had Roth judged the 12th round for De La Hoya, the fight would have ended in a draw.
Because according to *****s, Gayweather was ass-whooping Oscar. So why the need to reamtch. Someone mentioned BUM?
The biggest reason was no one cared. And Floyd's talks of rematching Hatton and DLH were seen as pointless and just a money grab.