Not sure why but I think it's because the other hours more closely resemble EMH than an actual park opening. Some things will be closed intermittently (for instance Splash closed at 4AM last time), but the majority of stuff will stay open for 24 hrs. I believe FP+ will also shut down at 1 AM so that could also be a factor.
There's no TS after that time. There will be some CS offerings open though.