For the same reason I can’t create a stream of 140 character messages or write Stairway to Heaven again. Because someone already did the novel thing and captured the initial audience attracted by that novelty.
No, you've claimed the UX was part of the reason they've triumph. And that is not the point, again, is not about the song (most of the songs use the same notes), it's about appealing to a certain group of people in the right time and taken it over. They've started with videos, like Vine, then they moved on to musical.ly style for youngers, and they merged. That's how they started and gained traction until 2018 where they took it off