Because they handled Covid much better and we didn't. However, they are now experiencing exponential cases like everyone else , but their cases are still much lower and death per capita are significantly lower with the US over 1,200 MORE deaths per million people than Japan (USA: #11 1,255 per mil, *** #135 38/mil) .
Even in todays escalated rise in Covid worldwide, the entire country of Japan at 126 million people has far less daily new cases of Covid at 5,446. Meanwhile, the US at nearly 3 times the population of Japan has daily new cases over 36 times greater at 200,000 cases per day. So, relatively speaking, we would need to have roughly 16,500 new cases per day to be on par with Japan. That's how bad of shape we're in.