His game, like Lleyton Hewitt's is much more suited for faster surfaces.. Clay requires a strong offensive game to stay on top of the point as great defense doesn't necessarily get you back in the point whereas on faster surfaces defensive shots can do so. Look at why Rafa is so strong on the surface, he is very unrelenting and consistently makes you play defense.
10 years ago commentators were wondering why Hewitt wasn't "dominating" on clay. You kind of summed it up.
Another way of looking at it is that with that kind of gamestyle one will better someone who's playing bad by inducing errors however against someone playing well it'll be nullified. Leaving the match in your opponents hands.
Hewitt's played some good matches on clay in his career. My favourite in a losing effort to Canas at Roland Garros in 2002. 4 setter in well over 4 hours. Canas leveled the match at 2 sets a piece in a 2nd set tie-break after squandering a zillion set points. Epic.