Tennis has always been an aristocratic sport, but that has gotten worse as the years passed. Now, space is at a premium, specially in big cities, and tennis courts take up way too much space. Housing and rent prices have skyrocketed. In the '80s you could afford to buy a house with a middle class income. The same houses that used to cost tens of thousands of dollars are now worth hundreds of thousands of dollars in some places. It makes so much financial sense to bulldoze tennis courts and build a big apartment complex in their place. If you live in a big city, it's hard to afford a tennis club membership. If you live in the middle of nowhere where you don't have these problems, there aren't enough people to play
It's not like the US has a shortage of talented athletes. The US still easily tops the medal tables in the Olympics. But it makes no sense for young athletes to invest lots of money into a sport like tennis where the payoff is uncertain. If you have to choose between developing into a world class tennis player and barely making ends meet (you don't break even until you reach the Top 200-300), or go into a different sport where you can earn a lot of money even warming the bench... The choice is easy. Of course, this also generates a vicious cycle. The lack of high profile American tennis players makes fewer American kids want to pick up a racquet, feeding into a further decline in popularity, and so on.
