"Why should a prisoner be disenfranchised as a punishment, in addition to having his/her liberty taken away?"

Well, mainly because we vote for our representatives who draw up and create laws. To allow someone who has made a conscious decision to ignore those laws, always at the cost of a victim, to have a say in the creation of those laws is, frankly, absurd. In criminal cases the vote should be denied prisoners - they have made the decision themselves to opt out of the system of public influence on the creation of the law, and the vote is the main way we are involved.