No good reason. Except perhaps that a perfectly implemented system (if such a thing is possible, so far none has been devised) would be faster.
Apart from that, the paper version is:
Extremely hard to rig on a large scale,
And most important of all: Easy to understand and audit by a layperson.
Many slashdot readers are well versed in CS and we do not trust these systems. How then can we expect the public to have any faith in these systems?
If the many CS guys out there really want to help, they should think out algorithms for sorting that are efficient, precise and can be done by humans - preferably with some error correction built in. It will not make anyone rich, but it may just keep a bit of faith in elections