Honest question: popular compared to what? How do you know? Because I've never heard of a game, which doesn't mean much. Meanwhile we know that millions of people are playing 5E.
In one of those state of Roll20 reports there was a graph that showed older editions of D&D were more popular than many recent games, even the other "big dogs" in the industry like Call of Cthulhu and whatever the new licensed hotness was.
On a related note I also think that it's quite possible that the people attracted to and playing OSR D&D are simply more committed and have a different attitude towards the game. Even if we accept that more OSR players are willing to DM than 5E DMs, that doesn't necessarily mean the version of the game has anything to do with it.
Why intentionally ignore the one concrete thing we can look at when making a comparison? Assuming everything else is equal, the only thing we can look at is the games themselves. 5E compared to AD&D and B/X and other old-school favorites. There's no comparison in complexity and ease of use, AD&D and B/X are just easier to use, easier to play, easier to learn, easier to pick up, etc. From either side of the screen. Yes, there's a silly PDF floating around about how impossible it was to play AD&D because of all the contradictory rules compiled over the decade plus it existed, but that ignores that most people didn't bother with every new bit of rules that Gygax published. They didn't even bother with everything in the PHB or DMG. And B/X or BECMI? Even easier.
It could just be that people that play OSR are more invested. Or not. There's really no way to know.
I mean...other than asking them. They are here, now, in this conversation with you. For me it's the freedom, lighter rules, more lethal combat, challenge of the game, and ease of use from every side of the screen. It's also the players' attitude toward the game. It's easier to hack and mod AD&D or B/X than 5E. It's also easier to get players onboard with hacking and modding old-school games than 5E. Players of those games are already aware of the preferred play style of those games and are onboard with it. The only benefit that 5E has over the old-school games is the player base being larger. But the majority of those players are new to the hobby and don't have the same attitude towards the hobby that older, more experienced players have, and a lot (re: almost all) the institutional knowledge is skipped over.
I think comparing player types in this case is comparing apples to oranges.
I don't. I've run and played both for both player groups. The article about a lack of DMs in New York nailed it. Ben from Questing Beast nailed it.
My experiences with both groups are as follows:
The old-school players show up and are ready to game, have fun exploring, freewheel things, hack the system, mod the mechanics, go with the flow, understand tactical infinity, and generally bash or sneak by some monsters, RP with some NPCs, and just play the game. These players are fine with losing characters and they understand it's part of the game. This also generally applies to winning and losing. Winning is preferable, but losing is something to accept.
I've had old-school players laugh off a character dying 5 minutes into the game. Slap "Jr" at the end of their character's name and keep playing with a smile.
The new-with-5E players show up and are ready to be told a professional-level story, complete with professional-level voice work, smash the buttons on their character sheet, and follow the RAW if it kills every last one of them...but then it's almost impossible to kill PCs in 5E. These players are not fine with losing characters and don't seem to understand it's part of the game. This also generally applies to winning and losing. Winning is expected, losing is verbotten.
I've had new players tell me that unless we followed every rule exactly as printed in the books we weren't playing "real" D&D. This player only wanted "real" D&D and quit. I've had new players rage quite over taking 1 point of damage. One. Singular.
I'm fully aware this is my personal experience and not indicative of every new or old-school player. I get it. But it is my experience.