Why is shota/loli not allowed in /d/?

I understand that such things are to be posted on not4chan, but why? Why does not4chan exist? I don't really care one way or the other, I just don't understand why that is specifically singled out, especially given all of the other horrors that are posted there.

Wow, have I really been around this long?

They moved the boards off of 4chan's main page because 4chan was in dire need of funds and the most viable advertising method had a Terms of Service prohibiting any form of "Child Pornography", whether drawn, realistic, or potentially illegal.