I have seen the claim that the FDA pulled the bait and switch for a combination of reasons.
1> placate the hesitant that were saying there is no full approval.
2> Keep Pfizer out of court, they cannot be sued for EUA treatments or Childhood vaccines, but can be sued for adult only vaccinations.
3> Many of the laws about requiring vaccinations require full authorization.
Whether EUA or fully approved, Pfizer is indemnified.
Same for all vaccine makers.