You keep repeating this, apparently without noticing that you're contradicting yourself.

If the airlines would make less money operating at night, compared to operating during the day, it's a net loss. If you're moving capacity, and you have to lower fares, it's a loss. If you're adding capacity, you're not clearing up anything during the day, and you're adding to your costs (maintenance cycles have to run more often, staff has to get paid more etc.) -- and you're probably cannibalizing your daytime travel with your night fares. Another loss.

No matter how much you want to believe otherwise, this isn't because of a lack of startup airlines, or too many mergers, or anything else. It's because people don't want to arrive places at 3 AM. If there were money to be made doing so, someone would do it -- B6/G4/F9/NK all come to mind.