Short answer: cities are too far apart and the USA is large. However, how much funding is there to really implement the same thing that exists in Japan but in the United States? Also, is there an incentive for that in the first place? What about population density? Japan is more compact regarding their population density while that’s not the case for America plus both Osaka & Kyoto aren’t too far from each other (but Miami & Washington DC are distant).
wouldn’t being bigger make more sense to have bullet trains. the faster you go the less often you want stops.