Unlike Nadal, Hewitt thrived on super fast surfaces. He loved having a big server as a target in front of him and owned most of the game's best servers back then, even Pete, with laser sharp returns, chip & charge approaches or great passing shots etc. Nadal's long backswing motion and topspin is a disadvantage vs. big hitter on fast courts, whereas Hewitt's ROS was more like Djokovic. It is no coincidence that Lleyton's downfall collided with the homogenization of the courts. From 2000 - 2005 he made a total of 10 semis combined at the US Open, Wimbledon and the YEC. Just before the courts (and his body) started to change.
Add to that countless injuries, his physical and emotional decline. I guess that not just his body but also his mind burnt out at some point. When you spend so much emotional energy in almost every single match, you are bound to burn out. Finally the simple fact that even before he was done he ran into Federer, who was simply a much superior player overall. That said, I liked Hewitt and enjoyed many of his matches around 2000-2004 from a pure tennis point of view (liked his style) but also from an entertaining aspect. He was such a fierce competitor, he still is.