Because their first fight has been heavily scrutinized for his possible usage of illicit hand wraps. Margarito himself claims that he was unaware that his gloves were being loaded for the Mosley fight so how could he say with any certainty that he defeated Cotto with a standard wraps. Furthermore, Cotto fought him valiantly and deserves a rematch. Now that Mosley has exposed his deficiencies, Cotto has a template of how to fight him and conventional wisdom amongst many observers believe Cotto is the better and more skilled boxer who should prevail in a rematch.
Margarito is not going to get his license back in the States anytime soon. If he goes and fights in Mexico behind their back he might never get his license back in the States.
Judging by the reactions to aything margarito, he will be huge on his comeback. The curiosity factor alone will have plenty of people tuneing in to watch his fights, not to mention the Haters, who seems to be the largest group. Anyone claiming not to be interested in seeing him fight is full of ****. They will be the first one's watching in order to so him fail in his comeback.