The most plausible reason I can mention is the change in behavior of each poster who used to be active there. There are many bounty hunters who used to flood the altcoin discussion boards so far have started to change their habits in order to gain merit and rank up. They try to improve the quality of their posts and gain as much merit as possible - the goal is not only to rank up, but also to join the signature campaign that is paid in bitcoin.

The terms of the campaign have likely forced them to spread their posts out across multiple board including gambling board. As a result - over time they tend to post on gambling board and start to lose interest in altcoin board. I used to take the time to catch some spammers on altcoin board - but I'm surprised that activity has dropped off so much.