Saw a study a while ago (might've been in a book, wish I could remember where), based on survey results...
Essentially:
* For people who want status. Big things are status symbols (just look at this site).
* For people who want to feel safe because they're worried about accidents
* For people who want something damn big that will run over anything in their way and that people will move out of the way of when they change the lanes/turn the corner without looking.
Pretty obvious stuff... although, people miss the obvious counter points:
* Style, class and luxury confer more status than "big". Mercedes SLK or Pickup Truck, who has more status?
* SUVs and Pickups are generally not safe... they have higher accident rates, higher centers of gravity, lower levels of crash protection in the form of crumple zones or other impact absorbtion, less torsional rigidity and being weighty beasts they have higher momentums at the same crash speed (meaning a higher impulse on crash).
* The other person might have a hummer as well, then who's going to stop?