That's what they sell ad dollars on. People will generally watch for an hour or more. Just like ads in the first half of the superbowl are the most expensive real estate.
Also, younger fans have no problem staying up past midnight. I lived in the easter time zone growing up and through college and I never had trouble watching a monday night game to the end.
Playoffs used to just be during the day back then, but I understand the revenue aspect that the league wants. TV contracts are what's driving most of the money now. Fan attendance is a smaller fraction
