The top level players have played alot of holdem, there is a lack of high stakes holdem fish and overall there is less gamble in that game. They are also all really good at nl holdem so its just....not as fun, for everyone.
Omaha there is more action, more gamble in the game in general, while there is for sure skill in the game of omaha... like I said there is more gamble so its fun for pros to play other pros because of alot of "luck factor" in the game.