As someone who generally hangs out in Free and Casual, I tend to interact with people from those areas. Not speaking for everyone from either sub-forum, but from the people who I've talked to, a lot of them seem to be both interested in Arena and Nation (haven't met too many who took too much of an interest in Tabletop admittedly). 

From what I've seen, it comes down to one specific trend or pattern: they're intimidated. There really isn't much of a tutorial for any of the subforums like Arena or Nation, so it generally tends to scare off newcomers. Or at least that's how I take it. Personally, I love Nation (I can't write fight scenes worth crap, so Arena isn't in my interests), but stepping into a completely unfamiliar sub-forum.

Just.. yeah, no.

People just don't like to step out of their comfort zones.