I don't know if that was ever explained, but Brian's tall, and Jeep Wranglers are a bit up off the ground, depending on the year. Plus, it's American made, and IIRC, C/L stated that Brian would want an American car, especially a muscle car, which explains the hideously fugly Corvette. there was a scene in 109, I think, when Brian went to the ealership to get a new Jeep, and the dealer told him that he didn't want that, that "fags drive them." Which explains him driving it through the showroom window.