Probably the reason is the schedule renegotiation that the STB required between the host railroads and and Amtrak once the passenger delay metrics were known and published, under the rule implementing the metrics that became final in December 2020. Any enforcement action could not take place until the host railroads and Amtrak were running under schedules that were agreed to after the metrics that would measure passenger delay were known to all parties.
The upshot of the passenger delay metrics is it doesn't count train OTP, but actual passenger delay. Any passenger arrival over 15 minutes at their destination station, regardless of station (be it Chicago or Wolf Point) is counted for STB purposes as delayed. Note this is a metric the host railroads fought very hard against (they wanted train arrival at terminals or at least specified intermediate points to be the metric) and the one the RPA fought for.
The Starlight, SW Chief, and Crescent had already had their schedules restrung under this (there may be others, probably are, but I didn't notice those). The hit to the Crescent was huge, the impact to the SW Chief and the Starlight were quite minor. The hit to the Builder is somewhere in between.
Note that three railroads had to be involved, BNSF, CP, and Metra. None want to be the subject of any passenger delay enforcement action by the STB, so the schedule renegotiation was probably pretty complicated between four parties.