I'd think one is more qualified to address local challenges than global ones. Certainly all those people made their money focusing on the US market, not only because it's big and governed by relatively uniform regulations but because they understand it better than any other. For the same reasons, it's IMO far easier to be effective as a philanthropist closer to home. (This is not to say there aren't exceptions to this, just that there's a certain tendency for it to be true. Of course there's the somewhat-Rawlsian argument that you should focus on those worst off first; without addressing this argument in itself, I think you should also focus on those whom you can make better off most effectively and who in turn will likely do the most philanthropic work once their own situation improves, and at least for a US billionaire, US citizens sound like a great target demographic on both counts.)