According to Stargate (and backed up by every Star Trek franchise and bad knockoff ever created), successful civilizations share several characteristics.
1. Speak slowly
2. Move slowly
3. Never use contractions
4. Wear robes or other enveloping garments
5. Eschew patterned fabric
6. Are white.
This is proven by the latest culture to creep in a stately manner across my screen, supposed founders of Teotihuacan who are so awe-inspiringly pale that they make a redhead in the dead of winter look heavily tanned. They wear all-white garments that cover everything but their shins, speak slowly and deliberately, and use three words where one will do. And again: White. Very white. Very, very white. Just like, you know, the native peoples of Central America.
Or perhaps I'm reading too much into it, and it's just really, really hard to find a Hispanic, Asian, or African-American actor in LA.