Why? At least if Gible stays with Ash we'll still see it eventually in league battle or what not. If it went to Iris we wouldn't see it once she leaves the show.
That's assuming that Ash would use Gible in a major battle again, which might not happen. He could easily turn into the next Muk/Kingler/Glalie. At least with Iris we would see it in action a bit more and even in the next generation when Iris returns for her own mini-arc like Misty, May, and Dawn did before her. I just see it as recycling, which in this case is a good thing since Ash has way too many Pokemon and very little time to focus on each of them. And Iris is trying to become a Dragon Master, so it fits.