It's popular.
Because it's good.
It's taken the internet by storm.
Remember the rest of the internet?

Yes but imagen Pokemon were sentient, and you wouldnt KO them but kill them

Forgive my lazyness, but i wrote an explanation on it, and i would rather just link it than copy and paste it all, mainly because ill have to get the song links by hand again
www.reddit.com/r/smashbros/comments/3wm0na/vote_for_melee_gamefaqs/cxxolrp?context=3

Undertale, its a story about a book that fell under a mans bed, its his quest to get his tale back.

I'm gonna explain it as best as I could

backstory:
Undertale got base support from Tumblr and Reddit. There was a Kickstarter for it and over 50k $ was donated by over 2k donators. (The highest donator got even added in the game as secret boss So Sorry)
conclusion - game was popular before hitting Steam

main story:
Undertale passed Steam Greenlight and people could officially buy the game on Steam. That happened around Halloween which boosted even more of its hype because average users on FJ thumbed up everything skeleton-related. Undertale has 2 skeleton characters so skeletons+puns = thumbs at the time.

personal thoughts:
I thought it was overhyped but the reviews and feedback on the game was great, so I got it. It has really-really great soundtracks and storyline. Lots of easter eggs and that oldschool graphics. I think its amazing. It can and will be annoying to people who haven't played it / watched videos about it so torrenting is also an option since its only 150 MB.

Using google is hard.

but google wont tell my why it has taken FJ by storm

It's popular.
Because it's good.
It's taken the internet by storm.
Remember the rest of the internet?

sprite based indie game that has multiple endings based on whether or not you kill the monsters. I was interested in the game but the fans are making me hate it

well, if fanboys destroy everything for you then you are missing out on a lot( no OPM, no Fallout 4, No anime of any kind, no Overlord, no Cosplay, No Star wars, no Deadpool, no undertale)

to anwser Quantumwaffle question, undertale is the kind of game you should go in blind so ill tell you about the game play and you just to trust me when i say the story is great

Gameplaywise it is a Turnbase RPG, the selling point is that when you get attacked, you can dogde all attacks in a bullet hell minigame(not tohou lvl of difficult bullet hell)
So it becames a mix of dodging and attacking,or you dont attack. You can Spare every single enemy there is on the game

The game has 3 endings, Genocide Neutral True Pacifist.
I think it was on sale a while ago, its not really expensive (10$) and it doesnt take too much time to complete, unless you genocide you monster

Theres a difference between enjoying a series and being enthusiastic about it, and posting images of said series as a response to every little thing you see. It gets on my nerves when you join a group for, lets say OPM, and after you make a post you will instead get images of undertale characters.

I guess in a sense, look at what bronies are like, but dial it down. That is how it feels

so sounds a bit like pokemon, where you can either kill all the enemies, run from all battles, or a mixture of both?

Yes but imagen Pokemon were sentient, and you wouldnt KO them but kill them

She was just scanning the horizon