This is a good question.
There are a couple of things that immediately come to mind.
Firstly, I don't sleep enough.....not by choice. I'd love to be able to sleep 7-8 hours but find it impossible. My friggin maintenance insomnia often sees me waking up at 4ish in the morning. Never have a problem falling asleep but it's a rare event for me to wake after 6am.
Secondly, my family responsibilities mean my weeknight workouts are done after dinner. I'd much prefer to workout around 6:30pm then have a feed, but I have to help feed and bathe the kids, plus I wanna spend time with them anyway, so I workout when they're in bed. Not the best scenario from a training perspective, but family comes first.
EDIT - how could I forget....thirdly, injury prevents me doing OH press and bench press. This is a severe compromise that shits me no end, but at least I can still keep workout...would be horrendous if it was a back injury.