Sabernomics does a good job of explaining why Jeff Francoeur hit much better than he should have in 2005, and why it took so long for major league pitchers to exploit his weaknesses:

And why is it that minor league pitchers figured him out? Well, look at the incentives for the pitchers in double-A versus those in the majors. If a double-A pitcher wants to move up, he has to get outs. The best way to do that is to prepare for the guys you’re going to face, especially the best players on the team. These pitchers saw he liked to swing at everything—a friend of mine who watched him in high school said this was no secret then—and they stayed away from the zone without fearing the free pass. But for major league pitchers, Francoeur was just another rookie. Why worry about him when you’ve got to face the Jones boys? And that’s when Frenchy’s window for success opened.

Hat tip, Baseball Think Factory.