The problem that Reddit realized early on is that user voting is the engine behind the content aggregation. That aggregation is one of the main selling points of Reddit. The more users vote on what they see, the more information Reddit has for how to aggregate that content. That’s what keeps the front page fresh, that’s what keeps content moving up and down on the site. In a very real sense, the voting is the heart pumping blood through the site.
So it behooves the site to not give any reason for users not to vote how they feel. Keeping votes private was part of that. It is one of the most basic tenets of democracy: the only way to give people the freedom to vote honestly and frequently is to give them the privacy to do it.
The potential for retaliation against users, in any number of conceivable ways, far outweighs any benefits that come from making votes public.
The voting information also makes it insanely easy to automate mass blocking of any opinion under the sun. Nobody in this thread seems to grasp all the things you can do with that data to manipulate user interactions on this site. If you think troll armies are bad, wait till those troll armies have a shared automated block list of every single person that has ever downvoted them.