Supply and demand. A good reason to start a forum is when it's necessary and nothing else seems to do. Examples are; when a new game comes out and gets huge, when something very specific starts growing suddenly in popularity, when a tight group of online friends feel "public" forums hinder their interaction and need a place to "hang out", etc.
The Internet does not need another art forum, so no one will have a real reason to hang out on yours, so you have no reason to create it in the first place.