One of the criteria for FDA Emergency Use Authorization is that there's no adequate, approved, and available alternatives (https://www.fda.gov/emergency-preparedness-and-response/mcm-...) so if they started looking into alternative treatments it could have jeopardized approval of the vaccines.
You'd hope that if there had been promising candidates for prophylactic/therapeutic treatment involving off-patent/generic drugs that the regulator and the medical establishment would have considered them on their merits, and not systematically squashed them with bad studies and half-truths in favor of top-shelf Big Pharma products. It would be unconscionable for people in these positions of authority to have shown partiality towards the pharmaceutical industry in a matter like this which would have delayed access to treatment and which gives rise to serious concerns about patient safety, especially considering the difference in the amount of medium and long-term safety data that is available about them (months compared to decades).
