Create their own forum won't get much attention Imho. That's only make people joined on their campaign but there is no attracted people will come. Weekly contest is just regular way to attract people to come and play, i don't see it will attract immediately since it's just a normal thing on gambling site
Create a forum will require resources to handle many things. Spam, scam, accusations and the forum is big enough and many gamblers are already here.
This forum is perfect for gambling companies to advertise and if they are companies that accept cryptocurrencies, the forum is more than perfect to begin with.