Because, if Beckham is to be even considered to be on the England National squad, he has to be game fit and playing in a Competitive League, and the MLS is not a competitve league on the world stage, I don't blame Beckham one bit, Remember Country before Club!!! There is a World Cup next year, and if Beckham can help them (Three Lions) qualify, then go for it.
That is the only reason he wants to stay in Milan, Serie A is a competitive league, if you play and train in a lesser league, you become the same quality as that league. It is a fact of sport and athletics.