Focus on boosting comments instead of sheer volume of posts—people post specifically because they want answers, and seeing a long stream of posts with no responses makes them think it’s not worth bothering.

I think people’s familiarity with the askhistorians subreddit may be discouraging anyone without graduate-level academic experience from responding, but the subreddit didn’t start out requiring that level of rigor—it became more selective as the volume of replies increased. But even a non-expert reply can encourage clarifying comments from people with somewhat better knowledge who might not othewise feel qualified enough.