I know there are rules for crew when the midnight boundary gets crossed - if they were in hospital until the early hours of Thursday then it's possible they couldn't work that same night.
Also consider if the flight crew were also tied up, they have much more generous rest times, so this could have been a simpler resolution than flying out a standby flight crew.

Would you want to be on a flight operated by someone who'd had little sleep, and was then expected to work a 14-15hr shift back overnight? I know I wouldn't.

As others have said, at it's busiest there are boatloads of flights out of LAS into all major airports in the US with plenty of daily flights back across the pond.