Look at the whole picture. Last year, he was a big focus in the offense, and especially before Carmelo, he got the ball in a lot of "his" spots.
This year, they've got Carmelo posting up or isolating in his area, which overlaps with Stoudemire. They have Chandler who finishes around the basket but doesn't add any space to an offense. They have no real distributor on the team at all. Stoudemire's downturn has as much to do with his teammates as his greatest successes do.