Because his endorsement carries great weight. He has to wait until the people have given their input - which is why we have primaries. Once they have, his endorsement can move the people's choice across the finish line by steering people who may have preferred another candidate towards the one more Americans backed.

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store