I think it has a lot to do with how much information is on the club's website and how often it's updated. Google pays a lot of attention to NC4x4, but it's also going for accuracy. So if you search for "Carolina Trailblazers", it returns the CTB website as the first thing. There are some NC4x4 links further down the page, though.
With guys like the Trail Jammers or Rock Corps, the NC4x4 stuff ranks higher.
But for the clubs that opt to have their own section, there's also the question of how you use it. You could have a public area where you have a FAQ about the club, how to become a member, etc. That's where you talk about an upcoming ride or post photos from the last one. But then you could also have a private section to discuss member business. Or maybe request to have the link to your club's main website added to the forum description, but keep the forum private.
So it's not necessarily an either/or.