It's hard to carrot already-wealthy prisoners.

For average folks, sure.

I don’t know, I feel like there is much more room for positive incentives than negative. Prisons put people on a pretty level playing field.

Like, the well behaved could be served pork chops and peas instead of slop and mush.

Have you ever experienced prison yourself or through someone close? Prisons don't put people on a level playing field at all. It is significantly more comfortable to be rich in prison than poor in prison.