The issue is that ActivityPub, the protocol used for federation, wasn't designed with reddit-likes in mind. Upvotes are repurposed Mastodon "favourite"s, and boosts are Mastodon's equivalent of "retweets", but downvotes are brand new with the appearance of lemmy/kbin. One could extend ActivityPub to have a new thing for downvotes, but non-lemmy/kbin instances like mastodon servers wouldn't have any use for them.
Also, a not insignificant number of lemmy instances don't have downvotes as a deliberate choice.