Re: @AC It's not "why", it's "how"
I consider felons losing their right to vote an atrocity and not worthy a nation that once view itself as the beacon of freedom.
I'm OK with not being able to run for an office while you're incarcerated - but these people should still be able to vote.
I think there's an irrational fear that because there are so many incarcerated people (per capita, US is no 1, I think) they could all unite and vote for one guy ;-)
Personally, because the candidates are usually interchangeable and there are so many voters, I consider it to be game of "large numbers". If you throw the dime often enough, you'll settle for a 50-50 distribution, which is what happens during most elections.
But due to gerrymandering and the winner-takes-it-all principle, you end up with stable majorities anyway.