Mat Oxley wrote a great article about the lack of British riders in GP:https://www.motorsportmagazine.com/opinion/motogp/truth-behind-britain-motogp-and-world-superbike
You can extrapolate for the US, but the upshot is that neither manufacturing nor the US motorcycle groups (the AMA in particular) have done much of anything to promote motorcycle racing. Also, the US has above all else a car culture - NASCAR, indy racing, etc all have good funding, TV time, and corporate backing. Another factor is that in the last 50 years, the number of small dirt tracks where aspiring racers could cut their teeth has declined dramatically. The value of land, population, and pressure from other groups has all done a lot to make cheap racing, the kind needed by parents and kids, more of a rarity. And the cost of racing has gone way up. Even my local club racers will sometimes spend $1K on tires alone for one weekend, not to mention transportation, camping, food fuel, crash repairs, etc. Money is tight everywhere, and motorcycle racing is an expensive sport if you want to be competitive.
Despite all this, JD Beach and Briar Bauman came in 1-2 in last weekend's Superprestigio flat track race in Spain. Neither Marq Marquez or Brad Baker were racing this year, but Mr. Beach managed to beat a very competent field including Tony Elias and Johan Zarco.