Find a few other people passionate about the topics and get them to post actively on it with you. Even if it's just four of you, others will be far more likely to join and chat than if they only hear the crickets. Paid posting is another option, for the same reason.
Again, I agree that the topics aren't very common, so your chances are far lower than usual.
Just keep at it. Remember that starting up is the hardest part. From there, things get better exponentially.