That change in particular makes it harder for non-military fliers to qualify. The main factor appears to be a reduction in the availability of military pilots (the big reduction in overall numbers following the collapse of the USSR, compounded by longer commitments to service,) followed by an expansion of civilian operations. And then there outsourcing large portions of domestic operations to barely minimum wage paying regional carriers which started in the '90s as well.

There is an interesting data at https://www.nap.edu/read/5433/chapter/5#56

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