Bureaucracy, and scarcity. Personal freedom to buy it would be great if there were more than plenty for everyone, but right now they should be given to those who have a much higher risk of severe complications and/or death. Not even the US allows private selling right now.
When dealing with complex dynamical systems (such as markets), you don't get exact answers. You get general phenomena which require intelligence to understand.
But in this case, the answer's very simple, and in my opinion, very obvious: High-risk people value the vaccine more, and allocating a proportion to the open market gives them the option of paying for one instead of having to wait for government permission. This is a first-order effect, not even a second-order effect.
The second-order effect would have been that more people might have been incentivized to create a vaccine if you opened the door for more profits. Right now there are lots of individuals throughout the world who don't have access to a vaccine.
