simple human psychology
I'm sure this is the simple stuff that Captains get to make decisions on every day. The crew probably expected to sit tight for an hour then be off, then that hour went to 2 hours, but then at the end of the 2 hours something else cropped up "be off in 30 minutes" so sit tight etc. And at the end of 6 hours, and no doubt with the pilots talking to Head Office about options, they finally get off.
It's easy in hindsight to say they should have immediately deplaned, but I bet the crew didn't honestly think it would take 6 hours to set off again. If they had known that at the beginning I like to think they would have deplaned.