That's the environment it happened in, in which other things existed. Unless you can reliably repeat it it's hard to claim we know anything directly causative. There are obviously a lot of things that helped, but it's not easy to repeat the experiment to make a hypothesis into a theory.
So we know the reasons but we don't know the reasons? I guess I don't see the point. Most of the time when people say "We don't really know why 5E is successful" it seems like what they're really saying is that 5E is, at best, a mediocre game and just got lucky. Usually because of Critical Role and Stranger things, which were things long after sustained growth has already started.
We never 100% know why any cultural or economic trend happens. I just see no evidence that the previous couple of versions would have been able to take advantage of that general cultural zeitgeist. There was no significant cultural or technological change between the release of 4E and 5E. The difference was the approach to the game design with 5E having playtests and surveys followed by a good game that lacked the barriers to entry that 3.x had and the approach of 4E that gave it broader appeal.
No game can be for everyone. But 5E has been embraced over the past decade by millions of new players. That doesn't happen in a world where people have so many pressures competing for their free time unless it has broad appeal and people playing enjoy the game. The fact that it broke the sales trajectory of most previous versions is proof that 5E did something right outside of just environmental factors.