Cowher went to the playoffs his first six seasons. Six! That buys a lot of patience from a fanbase.
Rex had two playoff seasons before he crapped out the last two. And remember that his second playoff season was shaky, Rex at one point thought they had missed the playoffs (before being told there was a small chance for them, one they did capitalize on).
Rex hasn't yet earned the right to have consecutive bad years, it's unclear whether he ever will.