I wonder if the effect works backward - excellence attracts the best and remaining on top requires assimilating the best from all sources. Diversity essentially is a side effect of their paradigms and world views in terms of openness to new things and experimentalism.

On a related note successful Empires become more diverse over time - one of the few virtues of Imperialism and they need the edge to expand further while the most xenophobic ones tend to be shorter lived or more limited in their success. One insulting but true observation about national flags flown by white supremacists is that they are all flags of losers (Nazi Germany, Confederacy, Rhodesia).