The LA Coliseum is located in South-Central Los Angeles, the PR guys at the NFL didn't feel safe in playing games at night there and that it would affect attendance, even worse than it already was. The NFL didn't want pictures of a half empty stadium being televised across the country for one of it's marquee programs.
I think it was because of league blackout rules. Games are blacked out in the home market if they aren't sold out 72 hours before kickoff. The LA Coliseum seats over 90,000 and the Raiders rarely sold out their home games. Thus any MNF games would have been blacked out in LA.
You guys are both right. And what really sucked about the blackout rules and the Coliseum was, RARELY did they sell out...BUT they average around 55,000 to 60,000 fans per game(approx. 450,000 for the entire season). As opposed to USC games that AVERAGE about 77,000 per game. Scary to think that you can average 77,000 per game and STILL not sell out a stadium.