1) Why is it surprising that it is so wealthy? I have always thought of Nashville as a fairly affluent city...
2) Why is it REMOTELY surprising that it is so Republican?? Lol. It is a wealthy and white county in a moderate-but-not-huge metro in the American South ... those areas are nearly all Republican, and the ones that aren't are centered around very diverse and much larger metros like Atlanta.
It's an oddity in the sense that among the top 20 wealthiest counties, only 2 are GOP-leaning and among them is Williamson.
I'm not sure what data you were using, but I used this Wikipedia page that cited an American Community Survey from 2016. This is just semantics, but according to this list, 8 voted for Trump in 2016:
Forsyth, GA: +47.60% GOP
Williamson, TN: +35.50% GOP
Calvert, MD: +18.40% GOP
Douglas, CO: +18.10% GOP
Delaware, OH: +16.10% GOP
Hunterdon, NJ: +13.80% GOP
Stafford, VA: +9.10% GOP
Morris, NJ: +4.40% GOP
Just for good measure, Nassau, NY was decently close (+5.40% DEM). So, I would say the list is pretty split...? Echoing back to my answer, a lot of the Democratic counties on that list are in large and diverse metro areas (every single one is either in the NYC area, DC/Baltimore area, Bay Area or Greater Los Angeles). Of the ones not in those very large, cosmopolitan and diverse metro areas, every county on the list voted Republican. So, it shouldn't be
inherently surprising to you that a wealthy county would support the Republican Party, and it should be less surprising to you that a wealthy county outside of the biggest metros in the country would vote Republican. I mean, exit polls show Republicans winning the wealthy vote in both 2016 AND 2018, and considering they lose wealthy counties in the biggest metros routinely ... those votes have to come from somewhere, right? Throw on top that it's extremely White, in the South and (as Del said) mostly made up of married couples (a group consistently more Republican than the voting population at large), and it would be surprising if Williamson County WEREN'T very Republican.
As to your question for why it is so wealthy, I am not qualified to answer, but it looks like there are good ones up-thread.
