That'd be an interesting psychoanalytic question for Rowling to answer. It's probably not a conscious thing.
I'm thinking it's a cultural British/Scots "take care of your own" mindset. I'm betting she never really even considered having "non-locals" come to help.
And perhaps she even sees the modern-day UK as already being in the grips of a corporate state that is less evil, but just as repressive as that in the final book ... and the shape of the final battle maybe represents her wish that the normal British people would finally wake up and take a hand once more in their own freedom. (That would definitely be the Scotswoman in her....)
The problem with the non-locals is that they are over sexed, over paid and over here...