The NFL told Disney they wanted to move the better prime time games to Sunday night. Disney declined that package so it went to NBC. Disney still had MNF but since it would be given the lesser games they decided it wasn't worth putting on ABC.
ABC is a joke. Do they think they've got shows that would do better than NFL games? "Commander-in-Chief"? "Wife swap"? Please. I'd rather watch an NFL game between two losing teams that have no shot at the playoffs who are playing second string quarterbacks. Heck, pre-season is better than most ABC "entertainment".