Why would it matter if felons voted while imprisoned or free? We should not be incarcerating so many people that their vote has anything beyond a trivial, marginal affect. That is to say the real question is why do we convict so many people of crimes?
Hmmm. Hmmm? I didn't really think about incarcerated before. But I suppose if you were imprisoned for a law you don't agree with, the way to change it is voting.
By keeping prisoners from voting, you prevent law from changing.