There are various reasons:
Societal mechanisms - Lower levels of trust.
This is a common social approach - if someone shows themselves as willing to break the social contract (laws), they have lower level of trust.
In terms of franchise, it means they can't be trusted with the responsibilities of said franchise - as a concept, not necessarily out of ...
You mentioned this practice in relation to the US; the legal basis at least would be section 2 of the 14th amendment:
Representatives shall be apportioned among the several states according to their respective numbers, counting the whole number of persons in each state, excluding Indians not taxed. But when the right to vote at any election for the choice ...
From this answer on Why allow convicted criminals to vote?, @origimbo gave one reason to allow them to vote as being
protection against the following algorithm:
Win a legislative election.
Pass any law which disproportionately imprisons the supporters of your opponents.
conversely, one reason to keep them from voting would be to ...
It's definitely not applicable to my home country (the Netherlands) for the simple reason that our electoral system has no provision for it. As we don't have a district based system, the 'tools of the trade' so to speak are simply not available. Gerrymandering is tied one on one to a district system, so no districts, no gerrymandering. Also, since there is ...
So that you can suppress voting among a constituency that does not support you.
Look at the group of people that is disproportionately incarcerated and what are typically their voting intentions.
If you crunch the numbers, you will find that for every single Republican vote removed by felony disenfranchisement, you remove roughly five Democrat votes.