Because it keeps anonymity and allows the person to express his likeness or dislike towards a particular answer.
I guess at least its initial purpose was this..
Now some people downvote a particular profile out of hatred, if you feel something like that is happening, you can ask a Platinum mod to check your profile nd then (if true) the mods will keep an eye on the downvote abuser.
probably to avoid keeping hatred for your downvoters
and break out to a clash that you will never uptvote your downvoters.
I think it is pretty much cool to keep downvoter anonymity ,because this could make a beginner feel bad and want to quit