My guess is that, historically, if you allowed an aircraft to depart - you had no means of knowing where it was. So you had to 'padlock' them. We are not far off being able to track aircraft and then they might be able to run the return side of the schedule to get pax home. But I doubt the insurance companies will change their mind in hurry.
A ghood article here, I think, about the factors, not least the Brexit vote affect on the Pound.
Monarch: Four reasons behind its failure - BBC News