Because Canelo is a middleweight now and Mayweather has never weighed in for a fight above 151lbs and there's no way Canelo would agree to a 152lb catchweight again and I doubt he can even make 154 anymore. You expect a retired 42 year old boxer to jump up to a 6th weight class?
No of course I don’t expect it from Floyd but you bet canelo would jump at the chance to erase that loss and Floyd would make his 9 figure payday in a meaningful fight. Not another exhibition.