Ratings. Swimming (esp with Phelps) and gymnastics are probably the two highest rated Olympic sports in the USA, so the IOC was willing to adjust the schedule for NBC. Track and Field is probably more of a worldwide interest, so the US is not going to dictate the times on those. Track, being outdoors, is also probably presented better at night, so that's not going to mesh with US primetime.
Sucks the next Olympics is in London...no way to really broadcast events live from there (during weekday primetime anyway).
[This message has been edited by SJEAg (edited 8/19/2008 2:03p).]