Money. Just look at -900/175/190 rates in the major's contracts, not even factoring ancillary benefits of a B plan/profit sharing/pensions (limited)/ etc.
Don't spend any more than necessary. Furthermore, driving competition for your business at a discount is a bump to overall profit margin.
He who pays, gets their way. Large companies pay consultant groups top dollar to find ways to maximize profits, at the lowest cost. Easiest target is labor and a B/C scale (pure hopes of advancement) garners the highest rewards. "Hopes and dreams" are emotion pieces, and everyone will make an emotional decision IF all practical pathways are shut off. "Get in, get your time, get out" sound like a theme? It's the reality, that is slowly changing, but folks smarter than us are already plotting against it.
I truly think DL is ahead of the entire industry. They have good rates, good contracts (or working agreements to the non-unionized groups in their portfolio) and have a refinery due to save them a boat load of money. Other groups used BK laws to shed pensions in favor of defined benefits (reducing market fluctuation costs-smart to the outsider). Groups such as FDX and UPS have learned to find a common fleet with flexibility to adjust to market fluctuations, while saving on overall labor headcount, to maintain profitability in both directions of the economy.
The industry is "healthy" at the moment. Unfortunately, this is a cyclical industry and timing and mgmt practices rule the roost. If you (any pilot) can get into the early stages of a hiring wave now (just based on retirements), you will do well. A few years from now it may be the tail end getting hosed if you choose the wrong mgmt group (read:not carrier) in a cyclical downward pressure situation.