The reason is simple why it's not possible: Lets say you want your lemmy.onion talk to lemmy.ca. That works. Now lemmy.ca needs to push new content to your .onion instance. It can't, it doesn't know how to speak Tor.

What you can do however is have a Lemmy instance that is on the clearnet and also reachable through an onion address. That's perfectly possible. But it doesn't give you much over just going through an exit node.

I think the main problem is Lemmy's controls against spam are pretty limited, and opening up to Tor users is just asking for trouble and that would probably eventually lead to the whole instance being defederated if they can't get it under control. Which involves more user validation, possibly email validation if not more, and now you're kind of defeating the whole point of using Tor.