It's already hard to make trades for basketball reasons with the rules regarding money, a variable cap would make it harder. Sorry Houston, we'd love to trade you Billups, but your state's taxes are just too low and we can't take back that much dead money.
Also, I don't think the NBA wants roster moves tied to fights over state taxes. Mark Stein is reporting that, if Atlanta cuts taxes to curb high unemployment, they will be over the cap and they will send Josh Smith to Minnasota in a salary dump. Or "Well, Jerry Brown got his higher taxes passed, but on the bright side the Kings got another 2.7 million in cap space."
Most of the time the bigger markets have both higher taxes and more local ad money for the player, thus its often a wash. Miami is an exception. So are Houston and Dallas, but you have to live in Texas.