You don't watch much clay tennis, do you?
Basic reasons are that he doesn't know how to move on clay and he lacks a finishing shot and the surface being slow doesn't suit his game as he likes the bounce lower and faster.
He has had good wins on the surface without a doubt, he is not a hack on clay at all.
Right on the money- he has made the quarters of RG a couple of times and also the semis of both Rome and Hamburg so he has had some pretty decent results on the surface. He also beat Guga in DC on clay in Brazil in front of a very hostile crowd.
GWH has accurately listed his limitations on the surface. Hewitt is at his most effective when he can counterpunch using the opponent's pace giving it back to them. On clay this is much harder to do.