There are a couple of issues that may be fundamental obstacles to improving our long-term question volume, that we have to take into account when considering potential solutions to the problem.
First, many board and card games are relatively simple and open, as compared to video games, for example. By this I mean that the rules generally fit within a booklet, and all of the game pieces and cards, etc. can be examined by payers without even playing this game. This means that in most cases players can generally understand the basic functionality of the game on their own, and most rules questions come from either a lack of clarity in the writing, or genuine ambiguities in the rules as written. And if the rules are genuinely ambiguous, this site may not be the best place to ask for help, because without additional information answers are unlikely to be authoritative or definite. A major exception to this is Magic: the Gathering, with a 200+ page rulebook and about 17,000 different cards, and it's no coincidence that we have nearly ten times as many questions about it as about any other game.
Second, in my experience most people play relatively few board and card games. This would mean that if someone comes to the site asking for help with one particular game, there are relatively few other games that they can ask or answer questions about if they stick around. This can have the effect of limiting the breadth of the set of games we actually see questions about.
In addition, I think Magic: the Gathering deserves its own special mention, because it provides such a large fraction of our total question volume. Magic has a very large number of rule and card interactions that generate questions that are usually easy to answer directly and definitively with authoritative sources. This makes it basically ideal for this site, but we are still not necessarily the best place to ask those questions. There are at least a couple of places on the Internet where you can ask those questions of judges directly, which has the benefit that the answer comes directly from an authority, and is generally considered to be binding.