As well they should August 19, 2021 by Robert Huntington Administration plans to contest 'Remain in Mexico' court ruling - Roll Call The Biden administration said it will appeal a federal court ruling that it resume a "Remain in Mexico" program for some migrants. www.rollcall.com
Why not undo “Remain in Mexico” the way it was done?
Presumably because of the delay involved and the safety of the migrants. See, e.g., https://www.humanrightsfirst.org/resource/mexico-still-not-safe-refugees-and-migrants (from 2018 but I doubt much has changed since).