Because he loved Fatima so much and he was afraid that if Ali, peace be upon him would have married, Fatima would be harmed by that and she would be upset and this would in turn upset our PRopeht salla Allahu alaihi wa sallam which would nullify Ali deeds. That is why the Prophet salla Allahu alaihi wa sallam said that I am not making halal things haram but if he wants to marry someone else, he should divorce Fatima, may Allah be pleased with her.