There is no moral character requirement for voting. Many members of society may have poor moral character without being convicted of felonies, and their right to vote is never in question. Judging someone by their past actions, which they have paid for in the eyes of the law, is undemocratic.
Additionally, there is no proof that every felon has poor moral character. People may be convicted for any number of reasons: due to poverty or self-defence, or even being wrongly convicted. The criminal justice system is not perfect, and should not be used as an excuse to continually disenfranchise felons.