Matt had very quickly after arriving at McKinnley his freshman year, looked around and realized that it was perfectly set up to be a high school drama, and he wanted no part in that. And maybe he had made some poor decisions--joining the football team had been a risk and joining Glee had been a serious lack of judgment on his part, but there was very little he could deny Mike and he really did like football and dancing--but he tried to make up for it by not talking to anyone.
But by the end of his sophomore year, he realized that if he stayed he was going to be pushed into the foreground by force because Glee seemed to be the center of the drama, so without explaining his reasoning to anyone but his parents and Mike, he moved away and left McKinnley behind, sad to sacrifice his new pseudo-family but unwilling to become a main character in a high school drama.