There is only one purpose in linking to a forum here: advertising to get more members.
Meanwhile, when you link to a website here, you can receive comments and criticism, people can point out bugs, etc. It has a point. People with real websites would have a very hard time if all their threads were drowned in blatant forum advertisements.
It's very difficult to "criticize" a forum. Yeah, you can criticize the forum banner, but then just go to Fan Art and have it criticized there. Because the content of the forum is not created by you - it's created by every registered member - you can't just go "Oh, fix this member's spelling" or "This member is wrong when he says this".