Ethnicity
The first part of the military to be integrated were the sustainment branches so even when the rest of the military was open to all races, the sustainment branches had more minority senior leaders to mentor young minority Soldiers. Also the majority of minority veterans come from these branches and encourage family members to join a field they can use after the military as they did.
I've seen the numbers, this isn't just a perception.
It's only uncomfortable when people inject "racism" into everything. From my personal perspective, people identify with those who are similar be it culture, part of the country, city, state, looks, school, etc. That isn't racism nor segregation. Visually, it may look like it to those looking for racism or segregation. It's really identifying with familiarity.
I did it myself identifying with all types of people but from the Air Force Academy, people from Chicago, other service academies etc. I was more comfortable talking to people from service academies than to people not from service academies. I am more comfortable talking to military members than I am with industry professionals. I gravitate to people that I can relate to and we all do it. Go to any social gathering and see who breaks off into their small groups - people who have something in common.
Forcing diversity has its negative effect of constantly putting "racism" in our faces. As for types of jobs that appear predominantly one type or another, I haven't seen where the jobs are closed off to certain people. The jobs are open. The Marines recently had some females complete one of their job types.
It's not a bad thing to ask people to mingle to get to know people outside of our own familiarities or even to vie for job types.
Anyway, my two cents.
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