In my opinion Itzy have also reached a level of popularity outside of Korea above Red Velvet too.
I think when touring is back on we'll see that further with a bigger demand and bigger venues for Itzy too and probably Aespa.
Red Velvet have just never been able to find significant sized fanbases outside of Korea, maybe in China? But in others places they seem to be a lot of peoples 2nd or 3rd favourite group so that's always kept them at a level of huge popularity, but always kept them from taking that second step over the line to maybe 400-500 thousand selling albums, big arena tours, maybe a dome concert or two.
For me I think the real issue is, in spite of reveluvs claims, Red Velvet have 0 stan attractors in their group. They have great music, big talent, a diverse discography, top visuals and a huge agency backing them! Literally every ingredient needed to take them to the next level.
But they seem, at least for me, to lack that fan engagement. That ability to attract a strong core fandom in many countries. Maybe being all Korean has been a disadvantage in some ways too.