I don't know what the answer is to be honest but I suspect it is like most other things in life and it boils down to hard currency.
Part of it really *is* that typical LeBron answer. Sorry if that injures your delicate sensibilities or something but it is what it is. I don't think it is such a hurr-durr observation. The best American athletes play basketball and football. The best athlete in Serbia? He plays tennis. I sometimes wonder if people from across the pond actually realize just how big football and basketball are over here.
There is a channel on TV dedicated to ONE singular college level football team (Texas). There are separate channels for different conferences in college football (SEC, Big12, etc.). Look at what happens in this country every March for college basketball. It's a madhouse. And remember, these are just the MINOR leagues. Never mind the major leagues....
2012: $12 million; 2013: $16 million
2014: $23 million; 2015: $24 million
2016: $25 million; 2017: $26 million
2018: $27 million; 2019: $28 million
2020: $29 million; 2021: $30 million
This is what washed-up has-beens make in Baseball these days. (Pujols)
All that said, I still don't have a good answer for you. I live in the middle (no coast!) and tennis is a bit of a tropical sport so maybe I don't have the best perspective. You need good weather to play tennis. The tour follows summer around the globe for a reason. So my judgement of the sport, coming from the temperate midwest, is likely not a very focused one. Maybe people from Florida or California have a better perspective as I am literally the only tennis fan I know IRL. People simply do not play tennis around here.
I think this is also shown in the women's side. American women are doing just fine in tennis. We have our solid vets and our up and comers. Why? Money. An American female athlete can't do a whole lot better than tennis at the moment. We have a professional women's basketball league but it isn't anything even remotely close to the men's side in terms of money....unlike tennis which actually is thanks to Billie Jean King.
So there it is. I know I am expected to give an answer other than the tired old "LeBron + Money = No Soccer for us!", but I really do think that is a major part of the answer. The money isn't there because the interest isn't there. The interest isn't there for reasons I can't really explain though. I'd truly like to know myself.
Maybe we are all just dumb Americans who don't understand why a score of 0 is called 'Love' and we just want to eat a corn-dog and drink a watered down beer and be fat and lazy and stupid. Is that the answer we were looking for?
Sorry if I sound upset. I am upset a little. I love this sport and I really do wish LeBron played it. But... he doesn't. And in that light, I am okay with rooting for Serbs, Spaniards, Limeys, and Swiss.