They were losing money and the NFL wanted Sunday to be the "big game" which ABC didnt want. Also ESPN could get subscriptions then. Take wiki for what its worth
Despite high ratings, ABC lost millions of dollars on televising the games during the late 1990s and 2000s. The NFL also indicated that it wanted Sunday night to be the new night for its marquee game, because more people tend to watch television on Sundays, and games held on that night would be more conducive to flexible scheduling, a method by which some of the NFL's best games could be moved from the afternoon to the evening on Sunday on short notice. Given these factors, as well as the rise of ABC's ratings on Sunday night, and the network's wish of protecting its Desperate Housewives franchise (which they knew would be costly), on April 18, 2005, ABC and the NFL announced the end of their 36-year partnership, with the Monday Night Football broadcasts being moved to ESPN starting with the 2006 season. ESPN's ability to collect subscription fees from cable and satellite providers, in addition to selling commercials, made it more likely that ESPN could turn a profit on NFL telecasts, as opposed to ABC's heavy losses.