I think it's because Thank You/You're Welcome is more common in older people, and that saying "No problem" makes it seem as if there's a problem in thanking someone, I guess?
I certainly have seen some older people say things like "it's not a problem at all" after being thanked, meaning their efforts to help didn't put them out or anything. "No problem" is maybe a bit more shorthand/slangy I guess but otherwise it just really does not seem like it should be controversial at all to me.