If Mayweather agrees and signed a contract to fight JMM at 144 lbs. (tho he didn't made the weight for some reasons), why can't he agree to fight Pacquiao at 145 lbs.?

Cotto classified as a welterweight boxer agreed to fight Pacquiao at 145 lbs. (and made the weight). Why Mayweather can't do the same?

If 145 is not acceptable, should it be at 146 lbs.? That's Myweather last weigh-in weight.