And that, in a nutshell, is the appeal of 5e. 5e is the Cheesecake Factory of TTRPGs.
I don't mean this to be either an insult (for those of you hate chain restaurants) or a compliment (mmm, I love me some Cheesecake Factory), but just a working descriptive theory. 5e has worked so well, has been so popular, not because it is great, but because it is so perfectly acceptable. Let me go through some of the various reasons, and then, since this is already too long, let other people rubbish the theory in the comments.
1. It's D&D. This goes without saying, but it has the brand. Everyone knows what D&D is. It's practically a synonym for the TTRPG hobby - seriously, if you have a friend who still doesn't know what TTRPGs are, you can always tell them that you are "playing D&D" and they will have an idea.
2. It uses all the old tropes. The six abilities. Saving throws. Classes (including the classics). Alignment. Fireball and magic missile. It's easy to bring players back when there is a certain level of comfort. No matter what edition was the last edition you played, from OD&D to 4e, you can make the leap to 5e.
3. It's got combat, and it's got the rest. Some people love combat, some people love the other pillars. It's always amazed me that there are people that play 5e and break out the minis and the battlemaps, and others that play it with few combats that are ToTM, and everyone in between.
4. It allows for optimization, and not worrying about it. D&D has a history of optimizers and rules lawyers, and 5e can certainly enable that. On the other hand, with bounded accuracy and attunement and other rules in 5e ... there is no pressing need to optimize.
5. At any given table, players can be both really engaged and ... not so much. One of the key features of D&D that has always been appealing (and often unremarked) is that it allows players to ... um ... check out a little. Some players don't like to have to be "always on" or telling their backstories or interacting with the environment or creating new narrative hooks for the group. They just want to be there, enjoy it, and doodle and occasionally interact. Why? I don't know, but there's usually one of them in every group ... Derek. D&D's mechanics allow for players to choose options that keep them from being the center of attention, if that's what they want.
6. The recurring event. D&D can always work as a one-shot, but it's best as a campaign; even the published adventures (Adventure Paths) acknowledge this. That's part of the charm, as well. Sure, there are many better TTRPGs in terms of one-shots (IMO) that don't require the prep and the setup time of D&D, but D&D has the standard "go out, adventure, kill stuff, get stuff, level, repeat" that is the basic pleasure loop of TTRPGs, and means that the same group can keep coming back for the same social event.
I could keep going on, but the idea generally holds. 5e isn't a great game in any given aspect; I would argue that there are other, better TTRPGs out there for many uses (and I'd even say that in terms of pure "D&D" some people might prefer other editions, or even clones). But when it comes to disparate groups of people getting together and finding that sweet spot of play, I am envious of the way that 5e always seems to be good enough for a critical mass of people to play.