It might be geographic proximity to Siberia and thus abundance of research material, which drives some funding to do more research which then drives more funding, etc.

Stepping stone on the way to making Godzilla a thing...

Or possibly just an abundance of biology graduates who need something to do.

Probably because they are the two most proximate advanced nations to much of siberia where these mammoths are found. Also, korean and japanese universities might have scientific funding and research agreements with their russian counterparts.