C.E. Evans said:
It's sort of like asking why are Time Lords from Doctor Who so British. The obvious answer is that T.V. shows generally reflect the sensibilities of the country that produces it.
Absolutely. Another related question is if there's a United Earth, why are the vast majority of Humans in Starfleet Caucasian? After all, the largest racial group on Earth is Asian.
The real-world answer is that, since Star Trek is made in the US, the racial makeup of the cast reflects the population of the US, not the whole Earth.