It seemed as near a perfect solution as could exist. Criminals who were convicted of the most heinous crimes needed severe punishment, the new social order needed to look tough, but the punishment shouldn’t be permanent, just in case the conviction was wrong.
Thus, suspended animation. The criminals were effectively killed, but in the unlikely event they were later proven innocent, they could be unfrozen, given a check and sent on their way.
System worked well. Many horrible criminal masterminds were frozen.
Which is why, when they all woke up after society had fallen again, what followed was so unpleasant.