Why did it take until the late 20th century for Brazilian Jiu-Jitsu to develop and the Gracie family crush almost all other unarmed martial arts at the start of MMA, when humans have engaged in unarmed combat for millions of years and every major country has long lineages of specialized competitive martial arts and tremendous incentive to find martial arts which worked and quick feedback loops?
I have a few gut-level suggestions for this, of varying strength:
1. In actual combat the circumstances heavily outweigh variations in style, so in historic clashes there was no clear superiority.
2. I think the incentive to find unarmed combat methods that work is fairly low. It only requires a little training with a weapon to be able to reliably defeat unarmed combatants of whatever skill. In example, the U.S. Army does little unarmed combat training, even though it is tremendously popular. As it was put to me, "The winner is whoever's buddies show up first to shoot the other guy." Also, it has a high injury rate.
3. Following on 2, almost all martial arts training broke towards art and away from combat; they severed their feedback loops. We can see this criteria at work in modern MMA, where boxing and wrestling are popular components of training despite the fact that they are sports rather than martial arts - but they kept the feedback loop, and specialize in important skills for MMA fighters (punching and grappling).
4. Until at least the invention of radio, I am skeptical there was enough demand for such a thing to motivate experts to travel and test their skills in this way. Most martial arts viewed fighting with other disciplines as unnecessary and arguably dishonorable. There needed to be a strong incentive to overcome these objections.
5. Until modern air travel, traveling internationally was difficult and time consuming.
For the interested, here is Joe Rogan interviewing John Danaher, a first-generation blackbelt in Brazilian Jiu-Jitsu from Henzo Gracie's school. They talk about the development of the leg-lock, and there's some interesting commentary in there about the relationship between the format of MMA and different physical skills emphasized by different fighting styles.
It's also worth pointing out that MMA has adapted to jiu-jitsu. Grappling arts are integrated into the MMA curriculum and no longer strictly dominate. Fighters with striking-heavy styles can win by learning how to defend against takedowns and then just playing to their own advantage. At the present moment, it seems that Western wrestling techniques are possibly more valuable in the "metagame" than mastery of jiu-jitsu, because a very talented wrestler can put almost any opponent on the ground from a standing position.
For some more conversation on a similar subject, here is Joe Rogan again, this time with Ben Askren who is a former Olympic wrestler talking about sports psychology and a feedback loop wrestling teams have that MMA does not. The relevant section picks up about 22:95.
This is worth reading for the excellent review of My Little Pony: Friendship Is Magic.
At https://www.gwern.net/MLP specifically.
I'm glad you finally got around to watching it! I stopped watching new episodes as they were coming out around season 6, but would still catch up occasionally until about midway through season 7, where I've been stuck for a while. This seems like as good an impetus as any to make it up to the end of season 8.
One thing worth mentioning about fanfiction--which I originally read from Bad Horse but couldn't find the original source--is that one benefit ponyfic has over other source materials is that you can write basically any story in the Equestria universe, enabling fanfic 'about people' rather than, say, 'about wizards' or 'about vampires' or 'about ninjas' or so on. I could more easily find his claim that fimfiction is just better than other fanfiction sites, from a UI perspective.
But why would that be an advantage exclusive to MLP? One could say the same about the Star Wars universe, for example (and indeed, there is a lot of Star Wars fanfiction out there).
I was thinking more of Harry Potter: it's similar in that we have a pervasively magical/fantasy universe in which the main characters often are engaged in adventures and are special or elevated in various ways, but there's enough worldbuilding that you can easily imagine writing a fanfic simply about ordinary everyday life (eg about a Ministry of Magic clerk struggling with goblin negotiations or Ollivander experimenting with wand R&D tweaks - "I have not failed 100 times, I have discovered 100 ways to not process unicorn hair for wands"). The Potterverse is entirely flexible enough to let you write almost any story you want - just set it among muggles, or in America, if you need to. Even J.K. Rowling does it... That doesn't seem like a major driver of HP fanfics, though. And the MLP fics I've personally read do tend to draw heavily on the main characters or at least fantasy or genre tropes (eg that one based on Death Note, or the isekai Myou've got to be kidding). One counterexample that comes to mind is Twilight fics which discarded all of the vampire stuff entirely, and that niche spawned 50 Shades of Grey, but I never would've heard of that niche if not for 50 Shades, of course, so maybe it's not the best example. If we sampled fimfiction.net randomly (whether weighting by work, chapter, words, or pageviews), would we really come up with a lot of MLP fanfics which weren't either very based on MLP adventure/fantasy plot or based 'on wizards' or 'about vampires' or 'about ninjas'? I'm doubtful.
(I agree that from what I've used of it, fimfiction.net does seem a lot better technically than fanfiction.net, which certainly can't hurt. That thing must be running on 30 year old code by now... and I've always been irritated by their dumb little tricks like CSS for disabling copy-paste.)