Because his successes far, far outweigh his positives, obviously. To say that he was "not so" good outside the £5-10m range is nonsense - he brought in players for around a fifth of their true value regularly, at a rate of one every season he was here at least. £2.5m for Arteta, a player worth at least £15m based on his time here and a player we sold on for four times his initial value after years of top service from him? The same price for Cahill, a modern day club legend, who was worth £15m+ in his prime and was a lynchpin for us? Nigel Martyn and Kevin Campbell, who were brought in and saved the club incalculable amount by keeping us up instantly?
Compare all that to players we actually didn't lose that much on at all, like Beattie, Bilyaletdinov and Kroldrup? Seriously?