The same reason they can't go to the store, hang out at the park, have a job...
They're in jail.
I don't think prisoners should be able to vote.
However, felons, who are not in prison, should have their voting rights and firearms rights restored. If you're not in jail, you shouldn't have to live like a second-class citizen, with limited rights, especially for things that are necessities for being a good citizen (ie voting, home defense, family-raising, etc)
Now, you might say "Well, what about sex offenders? They have to register and can't go certain places." The difference is that they can vote, and own firearms (some of them, unless they have some other case or some such situation in addition to their RSO case). They DO have an extra burden to bear, but that's the price of what they've done.
Voting should be universal to all FREE CITIZENS.