Many reasons. There's simply not a bike-friendly culture here, there are no special lanes, I'd have to drive in the road. Now I have to carry my lunch, gym clothes, etc. in a bag on my back, if it rains later in the day it's going to be inconvenient. Some days I need to go somewhere during lunch, or after work I want to buy groceries, then I'm constrained by how much I can fit on my bicycle.

Driving is just infinitely more convenient if the roads you're using are not congested.