Because more than any other team, the Yanks are not adverse to taking on salary, esp. at the trading deadline. Take the Abreu deal, for example---they didn't balk at taking on his salary in order to make a run at it this year.
Your overall point is quite correct, however. The Sox have a $125 million dollar payroll; for that kind of investment, they shouldn't roll over and play dead.
When you look at the Yankees, they have guys like Jeter, Williams, Posada and Rivera who have been with the organization their whole careers. Without those four guys, it is a much different team. Sure the Yankees go out and buy talent, but they develop quite a lot of it on their own, as well.