Maybe it's easier to turn bike commuting into a habit, because work commuting itself is already a routine. For most workers, the daily commute always involves the same schedule, miles, and terrain. Also, of all the things you could do with a car, work commuting is probably the most unpleasant, and the least amenable to improvement by changing your driving habits.

So, once you've got a few rides under your belt, to work out the details of your equipment and timing, then it's easy to keep it up.