Because we're booing a player who has been playing at a AAA level, if that, for a long period of time? Give me a break.
As I recall, the Yankees didn't win the World Series last year (I think we can all remember how much A-Rod contributed to that with his performance against the Angels.) So last year was a failure. What A-Rod did last season means nothing.
The fans aren't there to root for A-Rod, they're there for the team...something A-Rod is hurting right now.