Maybe not with complete certainty, but I think we can make solid theories.
Start with the fact that its popularity is largely due to the large number--a majority--of new fans it brought in. Sure, lots of folks came back-whether people who had moved from 4E to Pathfinder, OSR games, etc, or folks that had just stopped playing--but we're not just talking about a renaissance, but a cultural phenomenon. Meaning, if it was just us old-timers dusting off our dice, then we'd be talking about something closer to the 3E revival, or maybe a bit more. But we're talking about a new player base that is multiple times larger than that.
If it was just a renaissance, then we could focus on the game itself and what it does well (or well enough) to win people back to playing. We could look at its "classic" vibe that harkens back more to 3E and before than 4E. We could even focus on specific elements like Bounded Accuracy.
But because the immense popularity is largely due to the influx of new players--Zennials, mainly--than it probably has more to do with outside influence. Stranger Things could be the single largest contributing factor; I don't know the number of viewers, but it is probably high tens of millions, if not 100 million+. Then you have a confluence of other factors, some specific some more nebulous: Critical Role, the mainstreaming of geek culture, the popularity of comic book movies, celebrity spokespeople, convenience of gaming apps, as well as lockdowns a kind of general digital ennui.
So I think we're on solid ground if we posit that 5E is as popular as it is because of a perfect storm of cultural factors, most especially Stranger Things. Sort of like a soup: Lots of ingredients, but the "star of the show" (the meat, if you will) is Stranger Things. And because of that, it is less so about the game itself--whether the mechanics or presentation--and more about external factors. The game itself probably helped sustain popularity, but it didn't create it. To use another metaphor, it is like a rising wave that WotC was able to successfully surf on. But to be fair, I think the single biggest internal factor that probably had a significant part in maintaining that success, was the relative lack of glut.