For the same reason, if you're in the market for a used car you should check your nearest desert-area craigslist (e.g. http://palmsprings.craigslist.org/). I learned this valuable tip from a college roommate who grew up restoring old cars in Indio, CA.
And stay the hell away from cars from anywhere near the coast. I got my first used car from a lady who had kept it on the Oregon coast, and the thing was consumed by rust. Got me to bike races though, and considering the price, it worked well for the time I had it.
Cars from areas with cold climate where roads are salted for ice and snow fare worse too. The extra corrosion that develops on fasteners generally increases the cost of repairs, and makes working on an older car a pain in the ass.
The dry climate has another side-effect, of making it OK for collectors and car aficionados to drive their old cars around. It's not unusual in LA, away from the coast, to see '32 Fords ("deuce coupe") and classic hot rods from the 60s and 70s (like GTOs, Darts, and Impalas) being driven around casually.
