Why is Hellmuth so bad at cash games????

[cwsiggy]

We all know how good he is in tourney play. There is no question about that and he's having a good WSOP so far, but why oh why is he so bad at cash games?? What is the definitive reason. I believe I read once that Negreanu said he lacks a fundamental knowledge of the cards which I find hard to believe. Is is that difficult to be good at both??