Top online players play 6max because their postflop edge is maximized in shorthanded and HU play against worse players and the player base is smaller and smaller the higher you go so there aren't many full tables in high stakes usually.

I play 6 max and I find it more fun because the hands go faster and you can open up your game more. The main reason people play 6max, though, is they feel that they can maximize their $/hour by playing it. I wouldn't hesitate to change back to full ring where I learned how to play NL hold'em if I thought I would earn more there.