Karma might work on a per instance basis, but if implemented on a federation wide scale you'd have to trust every instance. It would be far too easy to artificially increase your karma with your own rogue instance just by editing the database.
I think it's mainly to discourage reposting for the sake of reposting. On top of that, lemmy doesn't go out of it's way to improve user retention, which I believe karma and the incessant notifications about your comments being upvoted, were about.