Yeah this is the real issue. Reddit's mass of niche communities, easy way to find them, and mod tools like matchday bots for sports and others really make reddit more appealing to a general market. I've seen users on Reddit comment on Lemmy and the consensus was "it's a cool alternative but too small" and I kinda agree.

Another massive hurdle is how entwined Reddit has become with Google searches. 9/10 any technical or research prompt I search has a log of Reddit posts with answers. This is no doubt the product of Reddit owners partnering with Google. I imagine it would be a real challenge for the Fediverse to replicate.