It was the light in the sea of post irony bullshit of the 2010s that was heavily gaining popularity like Dat boi and those shitty smear frame reaction pics , where sincerity was seen as "cringe" and videos shitting on other people's hobbies were at an all time high (leafy , h3h3)
It exploited the irony poisoned landscape of it's time to give us some of the most heartfelt stories ever seen from an indie game and became one of the most popular indie titles of all time
Long story short it became immensely popular because it came out at the right place at the right time , if it was released in 2016 I doubt it would have the same amount of impact as it did back then

Well it helps when third sanctuary is one of the best tracks I've heard from a game in a long time even from Toby , id rather have a track like that than fnf exe slop and meme shit in a romhack