More baseball

Entertaining NYT article on how Coors Field in Denver is hell for pitchers. Because of the high altitude and thin air, there is less resistance as the ball flies from the pitcher's hand toward the plate, and thus the pitchers are able to make the ball do fewer tricks. This translates into lots and lots of hits and drives pitchers crazy.

What I don't understand is, why don't they just raise the fences and make it harder to hit balls out? It's not like they're selling every seat, the way they did when the place opened in the early 90s.