"Diversity is not necessarily a strength."
The single most glaring factor of all is one that everybody overlooks, but it is one that maybe we should consider: those societies are relatively small compared with much of the rest of the planet and are basically full of people with the same ethnicity, religious background, and a host of other factors.
Basically, the fact that everything is relatively small and homogeneous in the population probably helps a lot in terms of trust, mutual respect, and general social smoothness.
It's a lot easier to feel charitable and socially cohesive when you're not wrestling against stereotypes against this demographic or that class and can actively see that everybody basically has the same values as you do.
Edit: Yes, I'm pretty much arguing that diversity is not necessarily a strength. It certainly can be, but not necessarily. - *