Probably because they aren't an authorized dealer. After you factor in the shipping and warranty, you could probably buy one locally for about the same, not have to send it back if there's a problem (swap in store instead), and have better service if needed.
I would buy locally but I live in a small town that only has two places that sell car stereo and they don't have anything worth buying. The closest towns that sell higher quality equipment are 175-200 miles away.
So to get what I want at a decent price online is about the only option. Thanks for the replies.