I'm surprised that it's only topping the charts now. You think people would be getting tired of it by now.
Finally Korea have their "world star," my students used to tell me that Rain was a "world star" and I had to break it to them that only the few people who were into kpop knew who Rain was and that he was largely a complete unknown. Years later, Psy has given them what they want. It helps that he speaks English though and can boost his profile by appearing on Ellen etc.
You need radio to be on your side these days to get to #1. It might have blown up first with the video, but it's only been recently in the last week or so that it's shot up on radio.
Re: PSY - Gangnam Style (South Korean smash single)
Well a fit guy like Rain can be found anywhere in the US. Psy become famous, one factor is that he is not like your typical popstar. but mainly he got discovered and raved due to the song with the riding horse dance.