It varies from site to site a little I think but in general:

The 2nd to 3d highest stakes are the toughest ones to beat. The players there have gotten beyond the newbee-state in their game and some are very good but they haven't build the bankroll yet to play in the highest games. This player base is much bigger than the ones moving up to the highest stakes and the more good players/fish the less EV. The rich suckers naturally don't wanna play for pocket change so they choose the highest games available. This implies that the highest games consist of a regular (pretty small) player basis and various rich suckers who gets in there and eventually loses money. I have no proof for this but I think this is the case.