Assuming that is not a rhetorical question, I guess for speed and accuracy.
How tough can it be to have a piece of paper on which you vote, and the paper is read and counted by a computer? If (when)there is a dispute, the paper ballots can be recounted.
In light of the reaming that some of those punch cards in Florida are suspected of having received, those old ‘fill in the bubbles’ cards seem to be a good idea. My kids did it in grammer school. If a person is not smart enough to fill in bubbles on a sheet of paper, perhaps they should not bother to vote.