The point in advertising is to expose the casino or the gambling related company to the users who are potential clients for the business to thrive. Advertsing in a Gambling forum that is not visited by thousands of users may not be worth. This is why bitcointalk matters for casinos that they pay a lot to be exposed here and not on other forums.
Actually there are several other forums that also advertise gambling and casinos as many other members have mentioned in this thread, but I agree with what you said, because basically advertising is one way to promote a product or site and the activeness of the audience is a very important thing it is important that the ad successful to achieving its goals, if we compare with other forums, bitcointalk is the most active forum so far and this is a reference for developers to advertise their products here.