The difference is that melee is easier to pick up for a new, average or casual player. So it is normal that in the leaderboard there will be fewer melees on 2400+ ratings, however, they will stomp casters on lower ratings such as 1800+. Casters will scale better on late patches when they get more haste, plus usual hardcore or pro players play casters more so they automatically will push higher in the leaderboards.