I'm guessing because she is the only real villainess in Batman's rogues gallery, and they wanted to keep her that way instead of having another antiheroine like Catwoman.
Not saying it's a good idea, and it looks like they may have changed their minds recently.
Personally, I like characters whose concerns are diagonal to the good/evil axis. Poison Ivy's primary concern is plant life: she does what she does to protect it; where those acts fall in regards to morality or legality is completely irrelevant to her.