Few things ruin a comic as much as retcons made by new writers because they didn't like something their predecessors did. What's worse, they usually make a big deal out of those changes and rub your face in them to show how much more interesting they made things.
In contrast, Stan silently and carefully changes a few things (humans, pets) so the comic is more coherent. There is no need to one up the guys before him, because he is the only writer.
It' not rocket science. It's also one of the reason why manga is more popular than capes.