In short, these are the reasons Pierce outplayed Kobe in the finals.
1. The Celtics defense was lightyears ahead of the Lakers, ranking as one of the greatest team defenses in NBA history.
2. Pierce had far less offensive responsibility than Kobe did, which coupled with the difference between the defenses made his offense much, much easier to come by energy and opportunity wise.
Kobe was the primary ballhandler, playmaker, and scorer for the Lakers. He had to do everything for that team. Allen and Rondo both helped handle ballhandling and playmaking duties for Boston. Because of the versatility of the Celtics offense, the Lakers couldn't focus on stopping one player like Boston could. We had to worry about Allen, Garnett, Rondo, AND Pierce, who each took over games in the finals. The Celtics only had one player they worried about - Kobe.
Pierce had more spot up shooting opportunities in that series than Kobe did the whole playoffs. Kobe NEVER had an easy shot because of both the defense and how the offense relied on him to do everything.
Pierce had a ty game 6 and the Celtics still blew us out - if Kobe had that kind of game the Lakers were finished. There was no equal talent on those teams, Boston had a 10 game lead on us and 3 HOF players along with a young stud point guard, and a bench that completely annihilated our's throughout the series. They were the better and more talented team through and through.
3. The primary defender on Kobe was Allen and Posey. Pierce for most stretches guarded Radman, while Kobe was on Rondo. Both conserving energy right? Difference being Pierce didnt have to guard Radman, who just stood around waiting for a kickout, while Kobe still had to chase Rondo around while he had the ball, even if he was playing off him. Kobe actually DID have to play defense on Rondo.
So its no wonder Pierce had the energy to guard Kobe at the end of those games, he had tons more energy both from having less offensive responsibility and less defensive responsibility. Along with having the rest of the alltime Boston defense backing him up.
4. Kobe did take Pierce at the end of a few games, only Garnett would come to set one of his famous "screens" and Pierce would be gone and to the rim. Why? Because LA's interior and help defense was complete compared to Boston's. Kobe gets a screen at the top of the key and Garnett/Perkins/Posey/Brown are all over him and he's forcing a pass to Fisher or Odom, both of whom the bed.
Yes, Pierce outplayed Kobe in the series. But there are numerous reasons why outside of "Pierce was the better player". Their team defense was astronomically better, and their offense was much less reliant on one player, not to mention the conveniently illegal defense they got away with all year.
Put Kobe on the Celtics and Pierce on the Lakers, and Boston sweeps LA by an average of 20 with Pierce putting up 2010 Iverson numbers.