I think it is similar to people who buy a new Dodge Challenger instead of a Nissan 370Z. The Nissan may handle better and be faster, but the looks, as well as the feel of driving a classic american muscle car (modern or old) is much different than driving a new foreign sports car. Some people just prefer that, even though a different option may be faster or handle better.
Harley Davidson is what most people think of when they think of motorcycles. It is basically an american icon.