I contend there are but TWO reasons.
1) President Trump touted it as a possible Game Changer. And, Terminal TDS causes them to reject it because of this fact alone.
2) The drug has been around for 65 YEARS. It is Dirt Cheap. Thus, there is no money to be made with it. So, Big Pharma is working overtime to discredit it and they are taking advantage of Terminal TDS of the Libs to help them discredit it.
I would add one more reason: their power and authority and egos are threatened. The idea that the drug might be effective was not theirs. Their worst fear might be that the public would ignore their decrees, and would be right to do so.