I'm asian and I'll give you my opinion
SK is a very homogenized country (much like your other East Asian countries like China and Japan where traditionally were the second and third sources of kpop idols) and therefore there is an element of "racism" involved
now it's not racism in terms of US' treatment of African Americans but less overt and more subtle
thus for any foreigner to be a kpop artist they would have to exceed not only the basic kpop criterion like dance and rap and singing but also other factors like culture and language and SK beauty standards etc etc which means it's a harder path for them to walk on