Why do you think Xiumin is so shy about his popularity?
He’s either very humble or just still insecure? It’s also possible that whenever he’s reminded of that he feels some crazy amount of pressure so he prefers not to think about it. Or some people have a hard time accepting compliments, no matter how sincere those compliments are, for whatever reasons.
It’s also possible that’s he really simply embarrassed at being singled out from the other members.
He basically went from being one of the least noticed members of the group and suddenly his popularity literally shot up overnight in SK when they started promoting for XOXO. I think he’s still trying to get used to that.
Regardless, I hope that he has some pride and comfort in knowing that he does have fans who recognize and appreciate his talent and presence.