Mainly because every series struggles to find the right tone or voice for itself in its first couple of seasons, and it's usually the third where the series finally plateaus and becomes more consistent in it's writing and production.
But I disagree somewhat, there are a number of shows that I think really took off for the better by their third seasons. Bewitched, I Dream of Jeannie, Hogan's Heroes, Sanford and Son, Cow and Chicken, Alvin and The Chipmunks, M*A*S*H, all really hit their strides beginning in their third seasons, though that's not to say that any of their first two seasons were bad, but in a lot of cases, they DID struggle, whether because of network interference (M*A*S*H), or budget problems (The Chipmunks), or simply didn't improve by then (really, Sanford and Son seems odd without the likes of Esther, Rollo, Grady, Bubba, et al).
But I disagree somewhat, there are a number of shows that I think really took off for the better by their third seasons. Bewitched, I Dream of Jeannie, Hogan's Heroes, Sanford and Son, Cow and Chicken, Alvin and The Chipmunks, M*A*S*H, all really hit their strides beginning in their third seasons, though that's not to say that any of their first two seasons were bad, but in a lot of cases, they DID struggle, whether because of network interference (M*A*S*H), or budget problems (The Chipmunks), or simply didn't improve by then (really, Sanford and Son seems odd without the likes of Esther, Rollo, Grady, Bubba, et al).