I just think the NFL has a lot more leverage over the networks. Clearly TBS expects to be able to show 100 commercials for The Guest Book as part of what they are paying for. It also seems the networks would rather lose viewers after 11pm than not have them (or not be able to charge pre-prime time ad rates) before 8. Especially when one game or the other is still outside of prime-time on doubleheader days to begin with.
It was actually funny to see Dodgers Stadium be so full the other night, it was so freaking late - and the game was so important - that nobody had to worry about beating the traffic.
And of course this is a very East Coast-centric topic. That's where the most people are, obviously. But if you live in Mountain or Pacific it's not a big deal, in Central the games still end at a reasonable hour, and Pacific fans already get screwed by 5pm starts, 4pm, would be even worse.
I believe Sunday Night Football also started in 2006. But yes, Thursday expanded and with the rise of the streaming deals I suspect they'll eventually find another night spot.