Even people who make mistakes should be entitled to vote. Even while paying for their mistakes frankly. They may have lost their freedom, but they are still citizens of the Republic.
The only compelling argument I know of is that voting in local elections is a mess because there would be counties that’d suffer from the over representation due to the location of the prisons. I would just consider those to be absentee voters myself, and they just keep the last address they had before going in or next if kin instead.