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False convictions happen. Especially if there's a sheriff or prosecutor who is responsible for lots of false convictions, it would be awful for victims to lose their right to cast their vote against that person.
Otherwise, there's an open highway for an oppressive regime that wants to try to gain control through imprisonment and the booth.
Their vote is a crucial form of negative feedback on laws that are too harsh or restricting, since many convicted felons will have a perspective on laws that others won't.
They are still in prison; allowing them the right to vote will hardly reduce their punishment. In other words, loss of suffrage rights is not the thing that deters potential criminals from crime.