My theory? Title IX is the culprit. 21 out of 23 players (and all of the starters) on the USWNT played soccer for a division I NCAA women's soccer university team. Because of football, other men's sports, and title IX, women's soccer is one of the best funded sports in college (to balance out men's and women's scholarships). There are 334 division I women's teams, with most giving full ride scholarships to most of the team.
On the other hand, Title IX has decimated NCAA men's soccer teams. There are only 205 division I men's soccer teams, and most of those only have a small handful (or less) of scholarships to offer. I've heard the argument many times that men's university soccer wouldn't make a difference because "other top soccer countries use a club system to develop talent from a young age, etc etc." I don't buy that though. Almost every single sport in the olympics that the USA is good at is because the athletes came through the NCAA system. I think if there were 334 division I universities that offered entire men's teams with full scholarships, we'd have a top 4 world soccer team in 10-15 years and never look back. The NCAA system is a juggernaut for developing athletic talent for national teams (most for the US, but many international athletes as well develop in our system). No other country in the world can offer anything even close. I wish Trump (or Bernie or Pocahontas) would make an executive order giving an exemption from title IX to men's soccer for 40 years or so. I think it would make a huge difference. MLS would get much more competitive, interest in the sport nationally would grow, more boys would play growing up with the scholarships up for grabs, and it would just perpetually grow itself.