Many of the early synths came from America. John Chowning actually first licensed his FM synthesis patent to Yamaha so that's probably what gave them a tech boost they needed to grow in the market.

Early electronics from Japan were often seen as inferior products compared to the American and European products (RCA, Westinghouse, Philips, Telefunken, etc). Japan of yesteryear was the China of the 1990s and early 2000s, some decent stuff but a lot of cheap clones. Now, China makes everything so that analogy falls apart today. The 1970s oil crisis was one of the major events that helped bring Japanese goods into a better light, Japanese cars were absolutely killing the domestic automakers on fuel efficiency and price and rather than engage protectionist tariffs (Chinese EVs today, hmm?), they forced the domestic automakers to try harder. This change in public perception helped give a real sales boost to other manufacturing sectors, like consumer electronics which in turn helped drive innovation in a very short time period. That's not to say Japanese product was junk in the 50s and 60s, there were a few dominant players of quality products (Sony) but like Chinese manufacturing today, there was still a lot of cheap junk.