The problem with modding, or really leadership roles in general, is that everyone says they want to do it, but very few of the people who say that will actually put in the necessary time, and even if they do, a large percentage of them will suck at it. You want to know why there's so much mod overlap in the large subreddits? That's why. The people in charge of adding more mods would much rather have someone they know is dependable than risk trusting their community with an unknown. I agree that diversity in moderators, especially between subreddits, is a good thing, but it's much easier said than done. Finding, vetting, and training new mods is hard work.