That's because having a bad game for DP is a relative concept, as he can change the whole outcome of the game in a matter of seconds, with one backheel pass, one freekick or one touch. That's why substituting a player like that is never easy.
i think ranieri overuses him at times. I mean giovinco does not replace del piero when he is having a bad game he comes on at the last 10 minutes no matter how bad del piero performs