I would say money is probably the number 1 factor in the US. The amount of money parents put in to having even a mediocre senior lady is huge, and now the skating has declined in popularity there is very little payoff. No college scholarships (I believe Miami of Ohio does offer scholarships for Synchro), very few skating shows, maybe a cruise ship if you are lucky and want to commit to that, or coaching. Coaching isn't even terribly lucrative. If you are a top level coach, sure, you make a decent living, but you aren't making millions coaching.
Times have changed. No the Kwans weren't wealthy but Ice Castle paid for her ice, her housing on the grounds, provided equipment, etc. And it wasn't only Kwan, it was offered in varying degrees to many National skaters Novice and above. Coaches were also able to secure private sponsors more easily back then.
Aside from money issues, the US has had bad luck.........Gracie was top level talent, but didn't have the right mindset. Skaters like Wang, Miller, Hicks, Glenn, Zadawaski etc. never lived up to their initial promise. Other girls like Tyler Pierce and Tessa Hong and Barbie Long have left the sport. Zhang grew and was unable to hold on to her shaky technique. Meissner got injured after winning worlds. Polina had momentum and won 4cc's then got injured and was off the ice for a very long time. The USFS letting Paganini go was a big mistake IMO too.
But all this being said, the writings has been on the wall for a long time. We used to dominate the JGP in ladies with middle of the pack girls like Alexe Gilles, Angela Maxwell, Kiri Baga, Becky Bereswell, etc. When's the last time we qualified someone to the JGPF? Polina/Karen (Wang from the US was called up when Chen wd) in 2014? You never know what will happen, but I haven't had a 'wow this is a star in the making' moment with junior ladies since Gracie.