This will be a bit of a compilation of different reasons I've heard by experts over the last couple of years, that doesn't mean it's correct, just some possible reasons.
1. The first is my theory, and that is that everyone is a winner, everyone gets a trophy, everyone on school tennis gets a certificate, and in USTA you get a trophy for losing, I've never seen anything so abusive to kids as this, that you are a winner by losing? That's child abuse in my book, for it's not real life. I have a company, and I can see that attitude with younger employee's, they think they are doing good just by effort, they are quickly dismissed. My son's played 4 years, singles varsity tennis each time, I asked him before we started "Do you want the easy way, or the way I was trained", he opted for the latter and we TRAINED, sprints, raised voices (An act I'm never angry at him and he knows it). So we go to tournament, I hear the opposing coach giving his prep speech, "What's the most important thing", "It's having fun"! This takes all pressure off the kids, allows them to not fight to win, they literally think they are doing great if they are just having fun, that they are winners, they aren't, their losers. My son whose 15 now jokes with me that in real life, you don't get trophies for losing, you get FIRED. So these guys, I mean come on Jack Sock? They might have talent, but they never went above their abilities because of this attitude, it's all over America, that somehow you are a winner by losing. I would like to see Nadals or Federer's desire in some of these American players, guaranteed they'd do well.
2. USTA not changing to slow courts, clay courts. We haven't caught up with the rest of the world, is it any wonder that the Spaniards, French, Europeans start learning on clay, and now dominate slow courts? Americans learn their craft on faster courts, then are thrown in with master slow court players, not sure if they can adapt.
3. American has turned into an entitlement society, people expecting things, in short we aren't HUNGRY. If I see someone from a poor country, they are going to be monster fighters, many of these guys train as if their life depends on it, because it does! It's either kill yourself in training and getting your game right, or be a poor tennis coach in Ukraine, not a nice choice, but this has always been true of every sport, those that are hungry, that come from poor countries with no escape valve do best, get the most out of their respective talent. I remember Arnold talking about Lou Ferrigno's comeback, he said "Lou has a nice wife, home, comforts to come home to if he loses, which is the exact wrong thing you want if you want to be a champion, you have to be hungry"
Aside from that I don't know, we must have a good talent pool, great population, certainly tennis is available. I'm not one of these nationalistic "Oh we need an American" kind of guys, but it would be nice to have someone whose good, Federer "seems" American, he speaks better English than many Americans and just seems American, but yea we've totally turned into a sissy society.