Interoperability is great and all but they’re two different types of sites and the majority of the posts aren’t directed at the community, they’re just mentioning them. They rarely generate discussion and the hashtags/links make for garbage post titles on here
Hashtags are kind of a Mastodon invention on top of ActivityPub. Usually, they show up as search links back the the website where they originated. For hashtags from other servers to work on Mastodon, the submitting server needs to set some special fields, and I don’t think Lemmy or Kbin do any of that.