[LT7A]

To the OP, Simichrome is a polish I've seen do amazing things. That said, I would agree with two factors mentioned above. One is that they are cast and probably a little more porous than wheels cut from a block of aluminum, and two, I think that the steel wool is leaving minute scratches. However, it looks to me like you've gotten great results already. Now that you've got them clean, if you polish them regularly and protect them with something that keeps them from oxidizing, you hopefully will work your way through those tiny scratches. So every time you polish them, they should get just a little bit brighter.