..... the same reason why people sell 1* sites? they either don't want to continue, have time for it or just to make money
I simple don't see why people with 4* can't or shouldn't sell their site, I think that established websites are often sold because it takes time to build a established site and within that time you get bored and once you get 'established' everything doesn't need your daily attention anymore so you its more fun to start a new one :P