From what Ive seen, it was easy to mod and just an easy simple game in general. With being so easy and simple, it allowed retards to easily put their ocs into the game and they eat that shit up.
The more simple, cartoony, colorful art style draws them in too.
The annoying beep music with no lyrics is easy to make and map out mods for and they dont have to worry about being good at music because the worst trash to ever meet your ears is considered great in the modding community.
Theres also the story: pretty much nothing, A guy gets into song battles over his girlfriend. Its impossible not to follow and is just a writing prompt for horrible edgy AUs.
The creator of the game made a joke about Pico and Boyfriend being ex boyfriends and thats when the community started growing. They later said it wasnt a joke of course, because they would have been "canceled" for half an hour otherwise. When you say theres an lgbt character in your fidget spinner of a game, even as a joke, youre doomed.
So, all it takes is a simple, easy game with bright colors, high pitched music that doesnt sound good, and a shallow story and you have yourself some perfect pastel flag waving retard bait. (Another example: Splatoon)
i wish they had set a trap to go with it, but instead they just kept feeding them... 
What i really dont understand is why it turned into this edgy "SpOoKy ChArAcTeR" kind of game, with mods for things like sonic.exe of all things. When that stuff started the only "spooky" thing was the lemon monster, which was hidden away in a debug mode.