MNF moved from ABC to ESPN I believe for several key reasons:
*Keeping the franchise under the Disney umbrella (since both ABC and ESPN are owned by Mickey Mouse and company) to prevent other channels/networks like Versus or Turner Sports getting a hold of it.
*ABC supposedly lost millions upon millions of dollars towards the end of their last NFL deal. This was compounded by still strong but otherwise, declining ratings. With ESPN, not only can they generate revenue from advertisers, but also from cable subscribers.
*A prime time National Football League television series doesn't exactly fit within the current image or mindset of ABC. A lot of their current programming seems to be geared more towards women (e.g. Grey's Anatomy, Desperate Housewives, Ugly Betty, Brothers & Sisters, etc.).
*The NFL realized that Sunday nights were now a more desirable timeslot to have the marquee game of the week since it's generally, the most watched day of the week, more people are at home (as oppose at work), and the flex-scheduling plays more of a factor on Sundays rather than Mondays. And of course, ABC is not going to bump Desperate Housewives for a dumb old football game.
*They hired George Bodenheimer to oversee the sports division back in 2003 and therefore, turn ABC Sports into an also ran (although in fairness, Disney was slowly, but surely intergating ESPN into ABC Sports anyway).
