Because both airlines rely on people going the "extra mile" to keep going, crewing doing extra shifts, ops doing overtime along with engerneering, cabin and flight crew doing all they can to maintain high levels of performance and service. With so many facing redundancy in the winter, especially in ops and crewing, how many do you think are going to go that "extra mile"? Especially as Thomas Cook has, rather cynically, just reduced its redundancy benefits.
Also, with people now fearing for their jobs a rash of resignations is expected over the coming summer, putting even more pressure on an already demoralised and over stretched work force.