75% of the answer is probably the covid retirements / massive hiring wave whiplash. United retired a huge group of senior captains and then needed a bunch of junior ones RIGHT NOW.

In particular they needed them in EWR and SFO, high cost of living, high index of frustration airports.

A senior WBFO (or even a very senior NBFO) could work the system and make something close to what a captain on reserve would make

United, under the previous contract, that allowed extensive (?unlinited) short calls and field standby. Many people avoided reserve at all costs.

UPA23 seemed to fix the problem, or fix it enough. Time probably helped as well as more people naturally came to a point in their life they wanted to upgrade.