Because Donald Trump is a pouting incompetent manbaby, the United States will not be participating in a global COVID-19 vaccine development and distribution plan. That's it, that's the only reason. The Washington Post reports that the Trump administration won't be joining the 170-plus-nation drive to jointly develop, manufacture, and distribute an eventual COVID-19 vaccine because Donald Trump is still mad at the World Health Organization for not being racist enough against China, so that's that.
Instead, the Trump administration is simply presuming that this America will, for some reason, get the vaccine first, have no problems scaling up supply, and certainly won't have any particular reason to share that vaccine with anyone else—despite the U.S. lagging in its pandemic research, if anything. And if those presumptions don't work out? Oh well. Sucks to be us. It's narcissistic delusion as foreign policy, yet again, and Trump's team is more than happy to risk as many American lives as there are Americans if it makes Trump not scream at aides for a few fleeting minutes of the day.
The decision against participating in a worldwide pandemic vaccination plan is Insane. There's no downside to contributing to worldwide vaccination efforts during a pandemic, unless you are an absolute monster. It's not likely to dent drug company profits—in fact, ensuring wider distribution would seem to result in more cash to vaccine manufacturers, if anything. Conversely, distributing vaccines to poorer countries will be relatively inexpensive, and especially so when cynically weighed against the economic costs, to the United States, of letting the pandemic ravage unchecked.
So this is just another incident in which Trump's racism and delusions of persecution are erasing U.S. national interests, entirely, simply because he wants to and has fired anyone in government who might dare point out to him that he is a foam-brained jackass. Trump won't pay the price. He and his cronies will get a vaccine no matter which nation develops it, and at whatever price they ask. Everyone who is not them, however, is on their own.