Because some who like that story want to believe
everything is perfect about it, from
every possible perspective. One thing I've noticed about many people is if they like something and someone says I don't like x thing about it, those who like it will often dispute it. For example, if there is something sexist about a show, and someone says they don't like the show because of that, another person who enjoys the show, and who the sexism
doesn't affect, will put out claims that the show isn't sexist.
The people who are not John Stewart fans and who don't really care all that much how he is portrayed may use
Rebirth as an example of Geoff Johns caring about the character, because, for whatever reason, they don't like John Stewart fans saying they don't like Geoff Johns' Green Lantern stories because of how he handled John Stewart. I would respect some people with that view more if they were more honest and admitted that we have a point, but they don't care because the portrayal of John Stewart doesn't affect their enjoyment. Instead, some try to argue that we're imagining things and John was portrayed excellently... which he obviously wasn't. He was just used to prop up Hal Jordan.
EDIT:
A line from John Stewart in
Rebirth:
"There are only two people I've ever trusted in this world. [...] Yes. Kyle. And Hal"
Ugh. This is sick, because John isn't really his own man or character here. He's just a part of Hal Jordan's world. He wants to be Hal Jordan's best friend, but he isn't even that, because there's Tom Kalmaku, Barry Allen, and Oliver Queen who have better claims to that title. Therefore, John can't even be the best friend of
his best friend.
Here is the same type of thing at play. John Stewart seems to only think of himself as what he is in relation to Hal Jordan, even in his own internal monologues: