It's easy to see why the game was made by a member from a forum dedicated to Earthbound: the influence is clear.
The influence of "tumblr/sjw" memes is clear here, too: the "hero" of the monsters? A "badass woman". You can date with characters of any gender (and species). Besides the king/queen, the only other couple in the story is lesbian. And several other small details in the dialogues (which work hard to almost always be "gender neutral"). Not that this is someting terrible, it's not something that the game rubs on your face; but it's there, it's basically impossible to miss it.
The game is essentially an JRPG with bullet-hell combat, wich is interesting in itself, even if it isn't mindblowing or anything. And it has a nice gimmick that can/will catch you off guard the first time you play, *exactly* because it's a game that looks, feels and plays like a JRPG (sans combat, of course). But... That's all. Is nothing deep or incredibly innovative, it's a shrewd idea that isn't very common in this kind of game - and people LOVE that. I think that this whole discussion on this topic about "what are the game X or Y success factors" is funny, because the very way of looking at this question is wrong. There is no magic formula with absolute values here: it's not the lack of ads, or the volume of it; it's not the kind of combat, or quality of the graphics. It's the a combination of these factors.
For every "rule" you will find exceptions. It's obvious that the volume of advertisements is an important factor in the success of a game - but even without advertising, games can be successful. It's obvious that games with a high level of difficulty are less sought by players - but hard games can also be successful. Obviously, outdated graphics decrease the sales of a game - but games with retro aesthetics can also sell millions. It's not about "luck", but it's a matter of a specific combination of factors can create an exceptional situation - just as people with severe and rare diseases, like cancer, can see their disease disappear for "no apparent reason". This, also, doesn't mean that there are no patterns anywhere ("cancer kills" or "difficult games don't sell" are, mostly, true), but as with everything else in life, there are circumstances that can create situations that are beyond expectations.
No one can categorically state what, exactly, makes Undertale a special case. It's a simple game, with a different combat (in a genre [bullet-hell] that isn't exactly famous), with an interesting gimmick that, deep down, it's anything too deep or innovative. But it presses the right buttons: a bit of feminism, a bit of 'mindfuck', retro art with good overall design and... BAM.
Someone is rich right now.
