Because 14x7's and 13x7's (even heavy ones) are becoming harder to find and more expensive, because final drive is open anyway, because running 16's and 17's on underpowered cars isn't a competitive advantage (just looks cool), because it cars to move from ITA to ITB without a wheel sale, because it will enable Spec Miata guys to run in ITA (great for 2 driver cars), because lightweight 15's are a dime a dozen (well actually $400/4)...

Who would support a change to allow IT cars to use any diameter wheel, as long as it was 7" wide or less?

Who cares enough that they would write a letter to the CB?