The common theory is a groin injury crippling his lateral mobility, combined with a reduction in pad size.
Jones isn't a very athletic goalie and relies on size and positioning, but his glove is a hole, he can't move across the crease quickly enough, he doesn't control rebounds, he can't read the play well and is not often in the best position to make a stop, and he's not much use with his stick (either pokechecks or playing the puck).
You're basically getting a 6'4" trash can on skates controlled by someone in the press box using a NES controller.