Mostly because of pisspoor managment.
The best example that I can give you is at Delta. During the previous contract the pilots took Draconian concessions on basically the last day of a recession. During the period of that contract the company went onto make record profits. When the pilots queried about the need for those concessions continuing and maybe they could just go back to the old contract Ron Allen responded "A contract is a contract" and that pretty much sets the tone for the industry. So now when managment asks for a concession, well a contract is a contract.
WHat happens in the employees make a concessions and invariably managment hands out bonuses that equal those concession. Had managment shared the wealth during the good times, then labor would be more receptive. Instead they have to fight tooth and nail to get back to where they were.
Cheers
Wino