This is an ancient question, with many different answers.. When Iwaku was founded back in the late 00s, there was no program similar to Discord, where you could have an entire server just to doing what you wanted to. Sure, there were some of us foolish enough to use Skype or AOL (lol). But as forums became a more centralized part of the anime fan fiction and RP community, the need for having an OOC and IC became more important. Many disliked having OOC posts right in the middle of an important scene. So it was considered rude. Jump ins were pretty popular for the newer members and some people still like them. The chat box also had RPers but when you don't have channels for it, it gets in the way. It's just kind of a natural evolution of the needs of an old era and many people following the same codes of ethics to this day.