I suggest to show downvoter's name so that we'll be able to decrease the number of unfair downvotes resulted by abusing his rights in doing so.
Voting is deliberately anonymous, both up and down. If someone wants to share their name with you or the reasons for their vote, they'll do so.
If we opened up the system to letting everyone see who cast every vote, not only would that feel a bit too social-network-y for my taste, but it would also severely compromise the integrity of said votes.
Maybe I don't want to vote something down, because I fear the user will vote me down in response. Maybe I don't want to be known as that one guy who downvotes Jon Skeet. Maybe I just don't want to be seen as a "negative influence" on the community.
Downvoting is important, and as long as it's generally warranted, we don't want to try and talk anyone out of doing it.
We don't want anyone feeling pressured into voting one way or another, because one of the main reasons the Stack Exchange model works so much better than classic forums is that people can trust content they see ranked high up.
And even on top of that, we already have enough cases of people overreacting to a single downvote. Giving them a target human being to overreact at would just make the site that much less enjoyable for everyone.
The only time when it's appropriate to call a person out on the votes they've cast is when they're abusing the system. And even that is held private under normal circumstances.