Sounds like we really need a, "What the heck happened to 5E" thread. So that we can explore how a poorly designed game with only intent to appeal to grognards became not just popular, but most popular D&D edition of all time. Leading theory so far is it was name dropped on a few TV shows.
I wouldn't exactly call myself a fan of 5e. It's D&D, and I have always loved D&D, but it doesn't suit me as well as previous editions. I feel like there's not enough customization, too few decision points, most of the base classes are a ghost town for new abilities, subclasses progress too slowly, and there are huge chunks of high level content where you go up a level, see what you get and go "meh". Add to that the lack of transparency about why some of the stranger rules are the way they are, and a lot of ambiguous stuff that either comes down to "well, the DM decides", which makes me feel like it's less a game and more a DIY project that I have to pay money for, or worse, some developer sends out a tweet (or whatever they're called these days) with their opinion, which makes zero sense, and sometimes I wonder if these people even play their own game. Lol.
But! I fully realize that for some people, this is ideal for them. They don't want lots of decisions to make, or tons of stuff to keep track of. The more the game builds their character for them, the better. Many DM's like the freedom of coloring in the lines (or outside of them) rather than have a concrete RAW to tell them "you can make changes, but you really shouldn't". And a version of D&D that's easy for new people to get into and for DM's to hack is apparently what's needed right now.
Media exposure certainly helped, as it raised interest in the game. Not only with Stranger Things, but the fact that D&D is everywhere on social media, TikTok, Youtube, Twitter/X, Reddit, Facebook, etc. etc.. You can find advice, "Let's Plays", podcasts, animations, stories- it's everywhere you look.
Add to that a new movie, tons of merch, and not only a decent video game, but a turn based CRPG that lit the internet on fire and won Game of the Year (even if video game awards don't really mean much, it's nice to have recognition). The sheer amount of "trapped in a world that works on RPG logic" anime is all over the place doesn't hurt either- people going on adventures who have levels and classes and belong to fantasy races is pretty standard now, even more so than when World of Warcraft was at it's peak (which was also a pretty interesting time for the hobby).
There's a level of validation to being a D&D player that just didn't exist in the past to this level.
Now reading this, you might come away with the idea that it's not 5e that's popular, but D&D in general. That any version of the game that existed right now would do well. And the thing is, I don't think that's true. As much as I think of this "70% approval rate D&D" as basically being bland like oatmeal, lol, all the buzz and hype would be meaningless if the game was built for a narrow demographic, like, say, "AD&D fans" or "people who bounced off Pathfinder".
The pandemic did help as well, as strange as it is to see something positive come out of it- suddenly people needed games they can play in their home or online with other people. Note that WotC and Hasbro had nothing to do with this, but like Amazon, they certainly reaped the benefits- they didn't even have a VTT of their own!
The big question now isn't "how/why is D&D big", it's whether this is sustainable. Will people get bored of the game's direction? Will Hasbro mis-manage the company into the ground? Will another contender to the throne arise? Or is the current version of D&D able to endure in "evergreen" state for decades, like it's Monopoly or something? Everyone has an opinion, but only time will tell.