In this case, the crash was very localized, with two big straights they could use to catch up.From my understanding of it the reason they don't do that is to keep the workers on the tracking safe. If they let them go around at the start they'd have to wait to send people on the track to clean up because the lapped cars would be going at almost race speed to catch up.
If they unlap, then let them get back to VSC speeds in the areas the crash isn't so they can catch up quicker or slow down the safety car a bit
