When the stars threw down their spears
And watered heaven with their tears:
Did he smile his work to see?
Did he who made the lamb make thee?
Democracy is us entrusting how we are governed to our citizens. We don't seek to force the perfect or best answer with democracy, instead we trust our citizens even if they get it wrong.
Prisoners are criminals who are not trusted and have been excluded from the citizenry.
Prisoners are not part of free society, are not trusted to be free and we have no reason to entrust our decision making to them. When they are freed and rejoin free society then absolutely that comes with the right to vote, but that right only belongs to our citizens.
You might as well be asking why foreignors shouldn't be allowed to vote.