I'm done work at 5, then I have a bunch of things to do, like make dinner, make lunches and extra meals for the next day, wash dishes, tidy up, do laundry if need be and then take some leisure time, so if I went after work I wouldn't get this stuff done in time to get a good night's sleep.
Plus I just have way more energy first thing in the morning, and I tend to lose motivation as the day wears on, as sometimes I'm on my feet most of the day.