None of us knows for sure but truth is a fair percentage of the population feels we can not return to any semblance of normalcy without it, medical professionals included. Could it be a bit of reverse psychology? Those who are against it aren't going to get it because Trump supports it but his detractors? Might they start to question it if he supports it much like they have with the various treatment options he's supported?

Then it's also possible that he knows something about the vaccines rolling out now that we don't. It's really anyone's guess.