I have had a thought tumbling over in my brain for the last week, and I think that I might be able to articulate why I ultimately prefer B/X over any other version of D&D or older games which I’ve played (Palladium, which likely doesn’t count), even if I will happily play any version of D&D, as well as many other RPGs.
Bear with me, please

. Some of this has been articulated by others, but I’m trying to give it my own twist. Also, please forgive me for going on a tangent, as the OP wondered why B/X was preferred over other olde school games.
The more abilities, skills, and so forth that a character has as a default characteristic, the less the character has to interact with the world around them.
If one has skills, they can just roll the skills. One is not required to describe how they would go about using the skill. It is often encouraged, but newer D&D editions don’t require it. Success is determined by the roll.
If they have a combat ability that trips, they can just use that, and maybe face a DC, maybe get auto success. There’s no need to check terrain, or footing, or feinting so your opponent is off-balance. Heck, there’s no reason that the DM has to describe these, as, again, the roll determine success.
The net result of these items is that the player is increasingly removed from the immediacy and impact of what an RPG can provide, which is essentially, “What if I were X in Y world with Z to do?”. That is an experience that you can’t really get anywhere else.
I am an introvert, and I’ve moved so much in my life that I do not feel that I am a part of any community. Combined, these make life a bit lonely. While I enjoy my alone time (my wife often says that there are days where I am too much company for myself), I also long to be a part of something greater than myself, and a place to be a part of.
Too much emphasis on character ability in a vacuum means that I will play just that, a character in a vacuum, and will just take the game as a theme park, floating lazily through Small World, briefly engaging with people in costumes, then going back home.
Without a great deal of character abilities and power concentrated in the character? Suddenly, I have to be concerned about my teammates, i have to pay attention to villagers, I need to make allies, I need to learn the laws, I need to learn geography. I become part of the world, even if the world is just a mega dungeon. Otherwise, I will quickly be slaughtered, have no friends, be thrown in jail, walk into quicksand, etc.
B/X is perfect for this, for me, as it has just enough to stuff covered by rules, just enough player power, and oodles of open imaginative space for me to “live in” with my group.
Anyway, you’ve heard people say, “Play the world”, right? I’m saying go beyond that, and be part of the world. For that, I think B/X is the perfect OSR system.