Hypothesis / speculation based on personal observation only:
MLP is similar to anime.
For various reasons, this demographic tends to be overrepresented among anime fans in western developed countries, partially (I speculate) based on the facts that watching anime requires 1) learning about it and being interested in this esoteric, nonconventional form of entertainment (this demographic tends to be more adoptive of nonconventional entertainment), 2) finding information about the anime shows to watch, which implies spending quite a bit of time on the internet for the vast majority of cases, and 3) actually finding versions of the anime shows one can watch, because for the vast majority of anime, if you don't speak japanese, you need to find a borderline-or-sometimes-outright-illegal version of it that was modified to add "fansubs", or fan-made subtitles translated in another language (e.g. english).
Most of the above is stuff that involves delving deep into the less-mainstream corners of the internet, or at least having a modicum of google-fu and the patience to learn the basic vocabulary of the genre.
Just that right there should tilt the numbers so that the base viewership contains a disproportionate amount of this demographic, and thus even if they were no more or less likely to enjoy it than other demographics, they would be much more numerous in its fandom.
It's not that hard to learn to find anime. You show up to one meeting of an anime club and five people will point you to AnimeSuki.
I recently published Mortal, a novella-length My Little Pony fanfiction meant to introduce anti-death concepts to an unfamiliar audience. Short description:
This is a character-driven melodrama. It's not particularly rationalist, but it's very, very transhumanist. Unlike, say, Friendship is Optimal, I wouldn't necessarily recommend this one to people who don't already know the source. It assumes familiarity with the characters and the world.
I am going to talk about how I put together the story and how people reacted to it. This will contain spoilers.
This line exists so you can break out of the automatic "read everything on the page" mode if you want to avoid the spoilers.
This story was structured as something of a bait-and-switch. I watched the reaction to a previous transhumanist horsefic (yes, there's more than one), and I was struck by how easily readers matched the explicitly anti-death narrative to the "immortality is a curse" trope. Rather than fight against this trend, I decided to work with it. The first act is meant to look like a story about learning to accept the inevitability of death. Starting in chapter 3, I break further and further away from that mold until the protagonists finally rebel against the status quo.
The first chapters got a lot of people invested who I suspect would've been turned off by a less familiar opening. Once I was into the third act, I stopped being subtle and used every trick in the book to make the pro-death characters look like the unreasonable ones. Judging by the comments, there's no shortage of readers who were angry at having their expectations flouted, but quite a few seem thoughtful, and some explicitly changed their mind on the subject.