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.

>>10
Not separate. Not4chan has no team. It's run solely by Saber who happens to be also one of people behind 4chan and it's on same server as some 4chan boards.