I'd say the biggest reason is culture and identity. The threadiverse is small at present - about 120k - and the microblogverse is bigger - 8m - but still smaller than the Threads.net 70m already and potential 1bn if meta leverages instagram. Why would a smaller and growing new independent social media platform want to be swamped by a commercial tidal wave? There isn't really a benefit to the independent parts of the Fediverse.

It's better for the independent parts of the fediverse to grow organically, remain independent and grow it's own identity rather than disappear into useless oblivion.

Also if I understand it correctly, the Threads.net is a microblogging site so while they may both use ActivityPub, Lemmy does not support microblog content. For Lemmy, it would mainly be the Lemmy content appearing within Threads.net. Federating with Threads.net is more of an issue for Kbin (which does both Threadiverse and Microblogverse content) and Mastodon (which does Microblogverse content) - the content would be visible in both directions. So for Lemmy it might be a big influx of users so may be manageable, but for Kbin & Mastodon it may also be a flood of content which might not be mangeable. But correct me if I'm wrong on that.