Well I moderated the Trade/Battle section at the time we abolished clans. It wasn't a one day decision either. Jake, the other moderators at the time, and I gave them a trial period to see if they would improve before abolishing them. We also made it a rule that you had to get Moderator permission to create a clan (similar to how tournaments work now).
However, they only got worse. The spam was out of control and no amount of infractions/bans seemed to change that. So we closed all active clans and made it so clans weren't allowed anymore. There was some anger for a while but eventually it went away and the trade forum is the way it is now.
Basically, they're too spammy and too much work for the staff.