Japanese players and Asian players in general can share knowledge and test bad match ups a lot more than people from the United States. West coast and east coast US players might play each other a few times a year at most. Top Japanese players for example though often play each other in person at many events, same with Korean and Chinese players.
People would also attribute arcade releases and Japanese players getting a heads up, but SFV never got a arcade release. Really comes down to a lot talent being close together and that raising the stakes competitively.