I think the Western rock bands also derive a lot of their popularity from the whole rock and roll culture associated with it. Whether it be partying, drinking, sex, the rebellious DGAF attitude, the aggression and non conformity, all of this has traditionally made rock stand out, give it a unique image and made a lot of fans gravitate to it. In addition to this, groups have also used rock music to sing about societal issues, the government, war, and a lot of other serious stuff in addition to good ol' rock ballads about love. Although rock music is getting more and more niche in today's age where EDM and hip hop reign supreme, it still has it's loyal fans and I'm willing to bet Korea has a good amount of them too.
However, these fans will mostly stick to those bands too. The K-rock bands we're used to seeing, are far off from what you'd associate with the rock scene generally, so maybe not too many rock fans check them out since it doesn't seem as raw to them. These are groups made by an agency, their music and MVs are also pretty polished, while rock loyalists probably seek something more rough around the edges. All these groups still have a target demo that they appeal to, but I think it's only the surface if we consider just rock music lovers in Korea. Most rock fans probably listen to indie bands, underground artists or stick to Western names.