To folks saying that 5e is so huge because it's either 1. an incredibly designed game or 2. because it's so heavily marketed/advertised ... I guess it can be a bit of both?
It's well designed! I'd hardly say that it's the best designed TTRPG though, and we know that quality doesn't directly equal popularity. Again, it's good- it has a lot going for it. As someone else said, it has a ton of momentum and brand recognition. That's not the only things you need, obviously.
Buuut if all the same influences that buoyed DnD's popularity over the last ~8 or so years had happened during a different edition of DnD? I'm guessing we'd see the same level of popularity. Stranger Things and Critical Role were biiig influences.
The suggestion that the quality of a system is reflected in its retention is a great point! Other things that can add to retention: momentum/popularity, investment, and walled gardens:
"Everyone's playing DnD, so I guess I'll play DnD;" "I have all these DnD books and I learned all the rules, I don't really want to buy and learn a new game;" and DnDBeyond. Holy crud DnDBeyond is a huge factor in retaining players, for multiple reasons but we really don't need to get into that again here