Right now my main reason is time. My schedule doesn't permit me enough time to get between work/school/home via bike and still have enough sleep. I don't have a problem pushing through the pain of sore legs, but not getting enough sleep is debilitating for my school and work performance. Thankfully my schedule will change very soon, and I planned more intelligently for this coming one since I wans't planning on commuting last year. This time I should be able to ride every day, and if I'm too tired I can just take the lightrail and do some homework while I ride.
I really feel for anyone who has to show up to work in formal clothes. Since customers don't have to see us, no one really cares so long as you meet a very minimal level of social appropriateness.