there is an easy answer if you knew game theory. but I'll try to explain. poker is a game of skill but is different from those games that depend on intelligence (like chess), talent and practice (like sports)... It's a game of incomplete information and some of it depends on luck, therefore even the most skillful is bound to make mistakes. Now all these make people THINK they can compete, but not everyone is lucky OR skillful enough. Or they are lucky but don't know where to stop etc etc.
And above that everybody plays against you (only except HU, there is one player to play against)... when you think of sports or other games, the opponent/s are usually equal with you/your team. So when every other person on the table tries to do their best to beat you, and the information is incomplete, and there is luck factor involved- it's actually a very hard job to maintain longterm success. Therefore you have only couple of choices to succeed:
1- make it a grind to minimize the risk- like it's a 9a.m-5p.m job. multitables, rakeback, and overcoming tilt issues will be main thing here.
2- luckbox a tournament.
3- be skillfull enough to jump through limits fast. (like durrrr etc.) this is the hardest one, as even though the play between the limits are not that different; the amount of money on the line always leads to mistakes. controlling emotions is key here. (also read "boywonder"s thread about this on 2p2 for inspiration- I can find the link if people would request)
This is what I think the problem is overall, nevermind the rake.