I can't answer for them, but I started with B/X. I think a fair amount of the attraction is that very brutality and procedure you mention. My experience is 5E players become dissatisfied with the ease of that game and start looking for similar but more challenging games. The players who also want something a bit more structured find OSE attractive. There's also a lot to be said about wanting to play a game that's a game instead of dealing with 5E as a story-focused game that a lot of people seem to push. It's also a self-perpetuating thing. OSE is popular because OSE is popular with a wide range of old-school gamers, so when new people come in, they are pointed to OSE. Like 5E with newcomers to RPGs.
This. And I'll add that over the 40 years we've been playing as a group (with new players and old players), we like the asymmetrical core design of the game. If you're out travelling, then there are random encounters. Those can be anything from a herd animal to a dragon. Sucks if you're 1st level (potentially). We grew to dislike the "everything should be about this tough so that a party of 4 can handle it" that leveled as you did.
We also like the procedure-based nature of things, as it makes the DM into more of a 'referee', for us, who handled what "fate" gave us. It was up to us as players to figure our way out of it using the tools we had to hand. Some of it is running. Some of it is clever thinking. Not that those things don't happen in 5e, but B/X and OSE are sort of built around a totally different set of expectations for the DM and the players.
It does work a lot better in dungeons and travelling point to point, but we're using it in Greyhawk sandbox, and we're okay with the pace of play (and it is overall slower: from managing resources, travelling, and a longer time to level up), compared to 1-16 in a month of game time.
My son, who learned on 5e, likes OSE much better. Simpler for him to DM, he and his friends can do ToM with less "worry" about all the rules, actions, bonus actions, reactions, etc. Or, at least that's what he tells me. But my son and I have also played 5e, OSE, Rogue Trader/Dark Heresy, and other systems as they've struck his fancy (like Vampire The Masquerade). I'm sure he mixes it all together when they play.