There are temporary promotions, there are permanent ones. In addition, in order to attract users and interest them in playing at this particular casino, you need to constantly add new promotions that should be actively advertised.
Yeah especially if the promotion is only for Bitcointalk users, it will attract a lot traffic because it just like a limited promotion and it will force other people to sign up in this forum in order to participate. When the promotion is available for every users, the chance to win is quite low or very low, so it's make users in this forum wouldn't participate. There are a lot casinos launch promotions only for Bitcointalk users and they're become successful.