Because it's not about who's better, it's about getting the best results under the cir stance.
If all things are equal, it's a no-brainer to start Parker; I was even a proponent of it a few weeks ago. But they need all hands on deck and playing to their best potential, and if you throw Tony into that lineup and it upsets the chemistry and continuity that's eluded them just about all year ... it's not all that wise.
Now, Tony coming back this early and George's injury could very well change all that, one can only hope (because you do want Parker starting and the team thriving around him), but if RJ starts to fade into the background again and the team's thrown off kilter at all, you can't simply go with him because of his talent; especially when his game could conceivably thrive and even benefit the team most, short-term (in the way it would effect those around him), coming off the bench to start. (I'd start him in the second half once there's been a rhythm and confidence built so that you wouldn't hinder his minutes too much.)
Like timvp said, it's not a slight to Tony or some kind of admission that George is a better player, it's simply making due with the cir stance and providing the best opportunity for the team to thrive cohesively.