I am a believer that in popular music there is an element of the "X factor" which is something intangible but to do with charisma/stage presence/force of character and that is probably exceptionally rare to find in pre-pubescent individuals and then to commercially market them beyond just a novelty factor - the real problem is distribution if anything
In classical music there is a slightly more "objective" character to performance given the high technical requirement and the audience culturally is more willing to earnestly listen to a child prodigy.