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Regarding theory B, the problem is one of promotion. I don't know too many people in that realm, and so I can't even go around emailing people asking them to participate. I pinged Luca Aceto (because I know of him from his blog) asking if he'd mention the site, and it may have slipped his mind. Of course even if he did mention it, there is the empty restaurant problem that Tsuyoshi describes. I always make a point of upvoting questions and answers in that realm if they make any kind of sense, but that's not enough.

For theory B to increase in participation, we need help from theory B folks already here to make a concerted push and get other folks to come and ask/answer questions. It'll take some time, but they should be able to acquire rep quickly. For example, one question on functional programming made it to Reddit and skyrocketed up the charts.

The same is true for other areas of theory A. For example, approximations, data structures, randomization and probability are areas that are heavily represented in the community but not here.

The best bet really is relentless promotion in forums where the people you're looking for show up. For example, target social events or other kinds of activities at conferences, or even request 5 minutes at a bisiness meeting to promote the site. In fact I was planning to post another question soliciting ideas for what to do at FOCS (I'll be going this year). Advertise nice questions, encourage people to post, upvote them heavily when they do come, and repeat.

Or come up with a splashy question that draws some attention. I don't know what that is for theory B, but I do think that the big-list/CW questions that we love to hate on meta actually might have one benefit in that they allow even marginal participants to contribute and get some credit (via badges if not via points)