Same reason I might care about the ethnic makeup of Japan. I like some aspects of Japanese culture and I would hate to see it disappear due to cultural homogenization.

I don't want to guess too wildly, but I suspect you also support the rights of indigenous peoples to practice their culture in the former British colonies like Canada or Australia? Why is the idea of wiping out British culture in 2025 OK, but the historical destruction of indigenous cultures - as in Canada's residential schools - treated as a grave injustice similar to genocide?