OP said why, not who.
Actually, what he said he wants is to see how many positive and how many negative votes, not just a summation. The current system already shows us this (it also just happens to include the who).
Even if you have 1000 rep you only get to see who voted, not why.
Most of the time, it's pretty obvious why.
The only useful reason for seeing the number of votes for and against a post rather than the sum is when a post has gained large numbers of both, which shows that the point is contentious. This rarely happens though.