Why are so many people against Undertale?

Because some people casually ky undertale, a yellow smiling face can be associated with "Wow, chara, how did you turn yellow" and so on, resulting in many people not having a movie viewing experience and so on.

There are also some very powerful guys who think undertale is more powerful than the dark soul.

"sans" of popular characters, you can go and see how many people brush him in innocent videos.