It's really not though. Relegation is the easiest stick to beat him with but people have mentioned his shoddy defences, the fact that despite media plaudits for "attractive" football his teams score a shockingly low number of goals, his pitiful win rate, that other managers did better in the league with Wigan etc.
We should forget that. There are hundreds if not thousands of managers who got relegated and stayed relegated and never did anything of note as a manager since. Focusing on the three or four who succeeded and ignoring the thousands for whom it was a fair representation of their managerial ability is kind of silly.
It's selection bias ... they are prominent names and we know their stories (more than we know the stories of the far larger group of people who got relegated and faded away). Michael Jordan got cut from his high school basketball team ... however it would be absurd to expect everyone who gets cut from their high school team will turn out to be the greatest basketball player of all time.