Three explanations strike me as plausible.
First, he believes that every ex-con deserves to "participate in democracy" regardless of how grievous his crime or how thoroughly it demonstrated contempt for law.
Second, he wants to increase the number of Democratic voters in Virginia to help his pal Hillary in the forthcoming Presidential election.
Third, he knows something.
It is some combination of two and three.
If he really cared about former inmates and their ability to "participate in democracy," he would have returned their 2A rights as well.