It seems like the primary reason isn't a buyout until it's approved. Linda P mentioned that any action will only be taken after MHRA approval, and MM might not be able to manage the surge once approval is granted. However, it's all speculation, and we can't be certain about anything yet.
Why would they be keeping price down for a buyout?
I believe it is primarily so the "too naked-short to cover" folks will minimize their losses. A buyout limits the shorts' loss to the buyout price, as opposed to a potentially much higher short squeeze price if one would take place.
If the naked-short position is on the high end, as large as 2 billion shares as some say is possible, then the covering process could have to take place over many months or years (speculating), to avoid a "mother of all short squeeze" situation.
But... upon approval - new-money buyers (retail, funds, institutions) will compete to buy shares already driving prices higher - the liabilities on the naked-short's books will multiply. It will have to be explained. If sniffed out, some wall street funds will pounce. Knowing someone has to cover, they will buy to force the price higher and force the shorts to cover, so they can profit.