Japan is the second largest and most homogenous consumer economy in the world, and everything works here (infrastructure, payments, etc).

That was the rationale that the cofounder of a different major Silicon Valley payments company told me when they chose Japan as the first country outside the US, and it seems like a credible rationale for Stripe to put Japan high on their list.

There's also a very low rate of fraud here in Japan, which should count for a lot.