Actually it hasn't. The DB didn't come into existence until 1972, and the market was steady improving from 1980 through 2000, and thus it never experienced a sustained stagnation. Having said that, here are the more important factors as to why "this time it was different":

1. Demographics - the pilot group was much younger then, with a much lower accrued benefit
2. DAL financial strength - there was no concern that the DB would terminate then, thus no reason for guys to bail
3. C2K - those pay rates coupled with the market down turn resulted in the liabilities growing exponentially, at the same time the assets were going down. Add in people cashing out by retiring while the assets were at the bottom, and the system became doomed.