There are more high level players on ICC because that's where the high level players are. Chess.com also has a decent interface, but they particularly excel in chess news, event coverage and community support.
That said there are plenty of verified expert-level players there, whom are probably at least 1 standard deviation above and beyond satisfying 99.99% of the world population of people looking to play chess.^1
> There are more high level players on ICC because that's where the high level players are
It appears this is not the case, anymore. I logged in to both servers this afternoon, and counted the number of players rated over 2350. Lichess had 50, ICC had 6. I logged in again tonight and saw roughly the same ratio.
Those data points could be valid, but it's important to note that chess ratings work relative to the players involved. A site that only allowed pre-schoolers, for example, would still have 2300+ rated players.
