I'm far from an expert on this sort of thing, but I would wager that the only voting data available through Reddit's API is the current number of up and downvotes, the overall vote score, and whether the account requesting the information has up or down voted, for any given post / comment.
It can do that because the data is centralised, and every account exists in one place, whereas federation has to say "cavalarrr@kbin.social upvoted this comment, and so did username@beehaw.org", because there's nowhere to store that data centrally, other than the post itself.