Good guesses but i'll tell you the real reason why Americans "suck" on clay:
First we have to admit that Americans suck on clay. Agassi, Roddick, Ginepri, Blake, Fish, Bogomolov, and any other American you want to name is not good on clay.
Next, it has nothing to do with not playing on clay. I trained for many years in Florida where every academy spends the majority of time training there players on Har Tru (similar to red clay with dirtier bounced..perhaps a bit faster)
American coaches (ESP the USTA) believe in hitting all forehands..ATTACKING..ATTACKING..ATTACKING. No one breeds a solid baseline game..no one encouranges drop shots..no one encourages a loopy inside forehand heavy ball to your opponents backhand. This is all discouraged. POWER..POWER..and Finish it off with a volley is the mentality of Rick Macci, Gary Kesl, Nick Bolletieri, USTA, Nike Tennis Camps, Evert Academy, Van De Meer, and any other place you want to go in America to train. Americans are good in tennis. We have very good players. But we do not have smart players. We do not have fighters. Hence, we do not have Clay courters. We have powerhouses..many of whom could be on steroids. With big forehands (Blake, Ginepri, Roddick) with big serves (Gimelstob, Roddick)
But we do not have clay courters. We will do just fine @ the US Open and @ WImbledon...but we will not do will at the French. Just accept it for what it is.
However Coria and Ferrero and Massu and Nalbandian refused to accept that. These so called "clay courters" made the final of Wimbledon and the US Open. And Massu went to the rnd of 16 this yr while Coria went to the Quarters.