I'm reposting what I put on the Sweden one, before this had a thread:
The drop-off in US men's tennis seems a bit mysterious. The US women have a lot more going, even in the up-and-comers, which makes sense, as tennis is probably about the most lucrative sport for women. But that tennis isn't over-all as popular in the US doesn't wholly explain the drop in competitive US men. Private academies and the USTA seem to be trying to fix this, but they have probably waited too long. It's going to take a generation or more to make up the difference. I'm holding out a lot of hope for Francis Tiafoe, particularly. But it will be interesting to see if the efforts bring a significant wave behind him and Donaldson.