It's just a matter of which country spread the language the most, I guess. But hey, there's no politically correct way of doing things here... They could use the flag of the "original" country where the language was first spoken... I don't know, they should replace the Spanish flag for the Castillian. And something similar would happen to all other languages... people would not be able to recognize the flags anymore.
As for the regional differences, well, I'm with Mace here. I hate when I get a "Spanish Spanish" instead of the Argentinean Spanish. Specially with movies, not so much for an internet site because we are used to the International Spanish (which is not International at all).
In any case everybody should be speaking Spanish, the most beautifully and graceful language of all. Period. We're a lot more than 5 million by the way.