Because they hadn’t gone all the way through proper trials. They’re just now in Phase 3 even as they mass vaccinate the public.
Hate to say it but Operation Warp Speed was a major mistake. A vaccine approval normally takes 7-10 years of safety testing so that long term side affects can be found.
Except they aren’t collecting data on side affects for a proper phase 3 trial
https://www.thedailybeast.com/how-pharma-cash-colors-operation-warp-speed-quest-to-defeat-covid-19
Moncef Slaoui, on the board of directors of Moderna, was selected by Trump to head up Operation Warp Speed.