I think one reason is because it is a rifle that the average guy can easily build himself, and they are almost endlessly customizable. I can't think of a firearm that can be configured in so many different ways.

The platform is just ideal for a "fun gun". Just think, you can buy or build a complete lower, and then buy all kinds of different uppers with different barrel lengths, rail systems, optics, accesories, and even chambered for different cartridges. You can shoot all of those different uppers off of the one lower versus having to buy an entire rifle for every different combonation. Oh yeah, and it only takes about thirty seconds to change them out.

Personally, I don't see what's not to like about the AR platform.