[lucasjung]

The multi-thousand dollar tax credits that made purchasing one cost-effective certainly drove its initial popularity (it's the only reason my sister bought one).

Even after the credits shrank to a level where a prius is no longer a cost-effective purchase, there are still U.S. laws (state and federal) which incentivise purchase of a prius (or other hybrid).

For example, a disproportionate number of hybrids are driven in the greater DC area. A big reason for this is because the state of Virginia allows hybrids to drive in carpool lanes with only one person (this may have changed--last I read there were lawsuits underway to challenge this policy). For people who live in the DC area, deciding where to live can involve some serious number crunching: the farther you get from the beltway, the less you will need to pay for housing, but the more you will need to pay (in time and money) for your commute. Buying a hybrid gave you a free pass to cut your commute time almost in half, allowing you to live farther away and save on housing.