From my observation it’s all about comfort. Having reports, especially ones with their own, who share your own background makes communication more frank. In other words, like a sibling, they all share the same state, it makes your job easier. It’s very hard to leave the comfort zone. I’ve seen leadership chain made up entirely white guys in their 50s because the guy at the top has a type. For me, mine are diverse but all are kids of immigrants or recent immigrants from Europe, Africa, Asia and West Indies. When everyone are qualified we tend to go with what we’re comfortable with. It’s unfortunate.