I really don't know, but I have some guesses that I want to share with you. The fact that ordinary loose tea never caught on in the public's awareness comes as a constant surprise to tea lovers. Somehow, there seems to be a barrier to putting dry leaves in hot water in places like the USA; this is so foreign. This is linked to why bubble tea does not suffer from this limitation; It is a flavored, processed, pre-made tea. I think the taste is just catchy. Sweetened black tea with milk added can be pleasant, but adding artificial flavors adds a lot of range to that and the potential for turning that sort of drink into a dozen different things.