Honestly, I don't think you need to blame diversity for this one. At least not entirely. It's definitely the cover reason why they're doing it but not the real one.
The real reason is what I personally term "mediocrity" syndrome. It's when you have people rising to power which they do not deserve. So, in order to preserve their power, they would simply favour underlings which would not out-shine them in any way. Happens the most in a stratified bureaucracy where one can easily control the promotion of individuals, since they are entirely at the whim of the leader. The Arab world is long notorious for this, and from what I can tell, the DNC is also suffering from this post Clinton and Obama-era purges.
EDIT: Come to think of it, this phenomenon is probably just indicative of a malaise in a polity in general. When meritocracy is discarded in favour of tribal loyalties and nepotism.