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.

May not be safe. The health benefits of biking kind of plummet if you become an asphalt decoration.