Why is Undertale so popular?

Answer 1

Because it makes the player care.

I could go into a long summary about how unique the combat system is, how wonderful the music is, how the author seemed to think of everything the player could potentially do, how funny the dialogue is, how impressive it was that this was an indie game, or any other of the well-deserved compliments Undertale gets.

But the bottom line is that Undertale makes you care about your actions in ways that no other game does.

A lot of games nowadays have karma or alignment systems, where you are encouraged to be an angelic goody-two-shoes or a psychotic jerk because those extremes net you the best upgrades, gear, and powers. But for the most part, you don't actually care about what you're doing; you just care about the upgrades. You know this game isn't real, the characters aren't real, and that ultimately, you just need to pick the option that you've been picking all game.

Undertale has a similar system, but expands upon it in a few ways.

First of all, the two paths do not play remotely the same. As a goody-goody, you will not be fighting people. You'll be talking to them, figuring out what they want, and making friends with them. You will spend the entire game at LV 1, with the lowest HP and no special powers. By contrast, the evil route has you gaining lots of EXP, upping your damage to the point where you can kill things in one hit, and providing you with some awesomely challenging boss fights.

Secondly, the characters in game don't treat you the same. If you're good, they will become your friend, help you out, and they will protect you from others who don't know how nice you are yet. If you're evil, they will conspire against you, and actually evacuate towns and shops before you show up, so that you don't kill everyone.

Lastly, the game itself doesn't treat you the same based on what route you take. It has no qualms about breaking the fourth wall, to the point where it will call you out directly for the decisions you make. In fact, at one point during the evil path, the game calls out anyone who might be WATCHING you play for being too cowardly to make these choices themselves. And so, the game reminds you constantly that you, the player, are causing all the joy or the Horror in this world. If everything goes wrong, it's your fault, and saving and reloading the game will not be ignored.

All this contributes to you actually feeling guilty for your failures, and triumphant in your successes. There's the old Maya Angelou adage of: People will forget what you said, but they will never forget how you made them feel. This game makes its players feel things that they're not used to feeling from games, and so as soon as they're done, they want to go out and tell everyone about it.