You only need to look at those companies that have forums to see just how little uptake game specific forums get. The warlord games official forum is a great example - For every active forum user there are 200 facebook users chatting on pages and in groups, all without costing warlord a penny.
If osprey are essentially just doing the publishing then I get why the games are sticking to facebook, Because it's the easiest way for an individual to cross pollinate and promote (although it's a bit oldschool now, Instagram is much nicer to work with and seems to generate more traffic, although still owned by facebook.)
Very very big games (like old GW systems) or older ones with dedicated fanbases (like battletech) can support a forum of hardcore, consistent posters, But new systems require new members, and keeping the chat confined to a specific place that people might not stumble across can cause a system to just sit in the void.
For all that I dislike facebook, It's a very functional platform for one man industries to make themselves appear bigger without taking on the added workload of forum management. (I currently run 3 facebook groups and 2 pages, 3 instagram accounts and a reddit account and it costs me nothing and takes very little work.)