Because even if she leaves USA Federation and skates for another country, at the worlds and olympics she still will have to skate against Tennell and Bell (or other ladies who are beating her at US Nationals), and she will loose to them Internationally, just like she did at Nationals.
... and here is another point, which i suspect some people will not agree with, and lets not pick on Alexia, but make it "general". If a US girl picks to skate for "obscure country A", and she is not as good as top US skaters, but better than "obscure country A" skaters, then she is taking away the chances from the girls born in that country. If there are circumstances where the whole family immigrates to "obscure country A" for political, employment, escaping various forms of disaster, then it is one thing. But one switched countries strictly for convenience of skating, then it is not fair to local skaters born there.