The Yankees local revenue function is just MUCH higher than anyone else's, combined with their own native resources. (this is one of the reasons that I hate discussing salary numbers without context) Whatever you put the luxury tax at, in general, the revenue per win that the Yankees face can close a business case for them. Now the Steinbrenner's might now WANT to put that sort of money into the payroll - like every owner, that is their prerogative. But the marginal wins that the Yankees can get by going from "mid 80-win contender" to "solid postseason contender" is well worth it to them in terms of the revenue streams and such. The Red Sox face the same sort of bonanza from adding wins, but have somewhat more limited resources. Increasing the luxury tax hurts the teams like Boston or Detroit or whomever that might be able to play in the deep end but can't.
For the Tampa's and Kansas City's ... they still can't afford what the Yankees can afford - but with the new CBA a lot of the tools they had to offset it (draft pick signing bonuses, international free agents) have been taken away from them. So the revenue sharing helps them somewhat, but it doesn't help them getting quality controllable talent.