My guess is they didn't care. While the ARC (the committee) was limited to 20 members, per the Committee Charter interested parties could request to attend the meetings (they were not open to the general public but my guess is that if United, the world's largest airline, asked to send someone the ARC would have allowed it) and all the notes, agendas, minutes, work papers, etc. were available via FOIA (or, more likely, freely provided to a company like United). In addition, the Advisory Report, which was issued September 30th, seemingly provides all the information needed to prepare a submission to the FAA.

I think this has very little to do with UA (willingly or unwilling) not being involved in the ARC but rather just UA ineptitude or UA not caring to invest the time or money to get whatever needed to get done in time for a Nov 1 rollout.