Why isn't Stripe operating in South Korea?

Regulatory restrictions and Korean banks not set up to accept Stripe payments.

Korea has domestic payment processors that likely lobby the government to keep foreign companies out.

All over Asia alternatives to American/western payment processing already exist, and local regulations may prove difficult or too expensive to comply with.

Americans don't have access to many Asian-oriented banking and payment providers, and other goods and services, so it works both ways. Sometimes it's typical protectionism through regulations and tariffs. Sometimes the market already has big established players making breaking into the market risky and expensive. I don't know specifically about Stripe in Korea, they do support it here in Thailand where I live. I know that no Korean payment processors are signing up new customers in the USA.

I would think that the censorship and other rules related to the internet are a reason for this.