Well, i don't really have a technical/scientific reason, but i prefer working out in the evenings.
1) I work full time, so my options are early in the morning or in the evening around 7 or 8.
2) Working out in the morning doesn't work for me because: a) Working out with a full stomach makes me very nauseous. Which means i would have to get up at least an hour before actually getting to the gym so i could eat and let it digest. b) I would always feel weak having only one meal in me, plus feeling a little ill still digesting. c) Lack of focus. I'm rushing to get done so i can get to work....thinking about the work day..etc. d) Aches and pains that work themselves out by the end of day.
3) Evenings, i've had a full day of meals, I'm more alert, stronger, blood's flowing, and the work day is over.
Overall it's really personal.
Some people are their strongest and most alert first thing in the a.m..