Why is it rated R?

Because they say MF a few times and probly some of the gore that they show

I know, man, Edge isn't even in it!

Okay, bad joke.

violence, strong language, and adult topics.
They talk about drug abuse (and show Jake the snake using) as well as discuss how he was conceived (brutal rape).
Not kids stuff.
And watching someone get their head stiched up, bleeding, etc. shouldn't be screened by young kids...