Well I think that is just what the forum member of the site are after .
Most of the member are surely doing it for fun or to interact with other members but their main reason would be to get free sats to gamble in PrimeDice.
It is a win win situation for the forum and the site, they wanted active players in the forum and their gamblig site and they are promoting it in the right direction so that there will be an active presence in both places and when high rollers are looking for a good site to roll, all they look is the sites reputation and if someone is new to the market they just look at the active users and is there any scam accusation and if that is fine, they are good to go.

It's bring traffics and popularities, when a site is highly visited, even those who are not really aware will noticed it when searching around the net, PD really did a great job maintaining their popularity and keep giving participants the reason to stay and keep supporting the business.
I think the reason why its a success is because the forum software used for Primedice is newer than Bitcointalk and its very friendly for mobile devices like the older iPhone 5 (5S) with smaller screens.
With Bitcointalk its very difficult to navigate and especially type on an old gen phone unless its got a massive screen like the iPhone 6 Plus or some type of tablet like an iPad.
Most people these days don't use a laptop even, let alone a desktop computer. Most people get by doing everything on their tablet or mobile phone. Hence why people prefer to use that forum since its more device friendly.