That I think is part of it, another is it's not really obvious whether it means 'save it for me' or 'show others I like this (and also save it)'. The latter overlaps with upvoting but then you're in a different kind of bind because you can't show individual votes publicly since you don't show votes. It ends up in a kind of conceptual twilight zone, even if the UI was clearer. A private 'like' and a public 'like' might just be a bit too much.