I understand that, given how Acela trainsets are operated today, only 8 Liberties would be needed to completely replace the Expresses on the Acela services. They could be waiting to just do a complete swap of trainsets on the service (i.e. upgrade all Acela schedules at once). Do note that the Acela schedules are drastically reduced from what they were in 2019, and that business traffic has not recovered the way leisure traffic has, so the full order of 28 trains would not be needed unless there is a currently unanticipated increase in time-sensitive (as opposed to price-sensitive) travel in the NEC.
Assuming a Regional train has 7 Amfleet I coaches, the Regional has 504 seats available, while the Avelia Liberties have only 386 seats (82 more than an Acela Express, btw), 8 regional schedules are diesel hauled on part of their trip, and not all Regional stations between Boston and Washington have high platforms so I think using Liberties for some Regional services would complicate both Regional fleet management, customer experiences, and schedules.