Why did Undertale get so popular?

Wiser men than I have attempted to crack the formula to success, with varying results. Simplicity, an intuitive interface, is useful for inclusion of the general populace. A compelling narrative is imperative to some folk, arguably the ones who become the most devoted followers. Hussie's endorsement I suspect broke the highly significant initial barrier.
Popularity's an exploitable system. I've seen a documentary about folk who acquired bot votes & comments, until reaching a certain critical mass where people began believing the content was popular enough to warrant following.