It's not my reason, it's just rumours I've heard on why clans are banned. It's nothing too big, but whatever. :P
From what I know (which isn't very accurate information), way back before I ever joined, clans were everywhere. Be it for Brawl or some other game, there was a clan for it, with just as many stupid clan leaders. I heard there was constant flame wars between clans, and just clan wars in general. I doubt it made the forum look very organized, or intelligent for that matter. Either the mods, or i0n himself (perhaps both), had enough and placed a ban on clans.
That's the rumour I've heard, anyways. Don't trust my information. :P If it is true, I'd say it's reason enough to ban clans, along with moronic clan members.