Because DC's awfully black and white in this respect. 'Jean's evil, now she's going to be punished'. That's it. Redemption, unless you're Hal Jordan, is a big fat dirty word, at DC.
I don't think there would be any redemption possible for her, really, at least with the current state of DC. If she did turn 'good' again, it would most likely be elaborate setup for screwing her friends over again.
The worst part is how it seems to have affected characters around her, like Ray, who, in the first issue of Robinson's long-awaited Cry for Justice, resorted to the fucking ridiculous method of extracting information from a criminal by using the same idea Jean used to kill his wife. That was a real WTF moment for me, and somehow, I can't really consider Ray a hero, anymore.