Here is why:
It is probably backed by insufficient data but it is possible
a) People often vote up a post because it was already upvoted by others. b) More importantly, people downvote some posts which have already been downvoted. The first piece of thought which hits me when I see a downvoted post is " What is wrong with it?".
Upvotes and downvotes have a natural place here; to show those answers first which have received the most number of votes and to eventually dim those posts which have received multiple downvotes. However, if the number of downvotes or upvotes is not displayed it will hopefully encourage people to read the posts more carefully before downvoting.
Please chime in with your opinion!