The problem is that the client (animus.exe) is closed-source, so it can only be changed by using a hex editor, so it's really hard to add new features to it (or more or less impossible for someone like me). Unfortunately, the client only understands a handful of message packets, like 42 (for monster/npc/main/trade chat), 31 (for public chat), 189 (party chat), 576 (guild chat), etc. So there's no way we can add a new one on the client side, so main chat and trade chat have to share the message packet number with one of the existing ones, since main/trade chat are not official, thus they are not included in the client as an option.
On the server side, everything's possible, so I could give main and trade chat a new message packet number, but that doesn't help much in that case, because the client doesn't know what to do with the new numbers.
PS: The numbers 42, 31, 189, 576 do not make sense, I just made them up.