Ive been thinking about that a lot too. My only conclusion brings the following points, the two first ones were already brought up:
1. He has a mental block for fast courts, he looks like he is not feeling well at all over there.
2. He runs out of power on the second part of the season (from RG on).
3. All that combined with some little details on his play like the ones I will point makes his confidence very thick when he plays on hard courts. It is easy to put him under pressure and make him kill himself on a match in that surface. If the opponent is smart enough to MAKE HIM PLAY and put him under pressure, his chance of losing is enormous.
Why does that happen? His play on hard has a couple of problems, very little ones, but that make him lose some points and his confidence sinks fast.
a. his between-points movimentation. He takes a lot of time to get back to position when he is being attacked on hard courts. A lot of players who prefer clay (I do feel that myself), has a hard time when they cant slide. You just get unballanced and everything takes longer. So, when the opponent is attacking him, he is usually NOT in the right position.
b. that point above makes his game suffer in some aspects:
first, he has to run a lot more. That seems to be the reason why he gets out of gas fast on hard.
second, he cant make the counterpunching game work off-balance.
third, he loses confidence when he cant feel the ball well enough.
fourth, his balls start to fall short.
fifth, he cant attack anymore.
Those are some points I can remember when I think of watching him play on hard court. It can help that Ive seen him lose many times in it, and most of them the matches were close and he just lacked A LITTLE BIT to win. Thats why those points seem to be so detailed. They are very small things, but they just crash his confidence.
Thats my opinion I guess.
I hope it makes sense. lol