Both Agassi & Hewitt were/are better on hard courts for different reasons. Agassi was better on clay later in his career because he became more patient in the second half of his career. He hit harder and through the ball more and benefitted from the faster surfaces because of that. Clay courters are more into spin and movement and working a point, court positioning and such. Agassi got better as he matured but, since he was a lifelong hard court player, he never mastered it like players who grew up on the slow stuff.
Hewitt would be more suited to play clay court tennis, but, like Agassi, it's not the surface he grew up on, and he doesn't hit with enough spin to be effective on it. Hitting flat shots is very ineffective on clay. The ball sits up perfectly for your opponents. Hitting with heavier topspin makes the ball jump more on clay than on hard courts.
Finally, as to your saying you should have posted this in the introduction section, actually you might think about starting a journal post in the journal section. That may be where this is better suited.
Don't get me wrong, I'm not criticizeing your putting it here, just responding to that comment.