Wikipedia:
“On December 8, 2018, President Donald Trump announced that he would nominate Milley to serve as chairman of the Joint Chiefs of Staff, although Secretary of Defense Jim Mattis and then-Chairman of the Joint Chiefs of Staff General Joseph Dunford favored Air Force Chief of Staff General David L. Goldfein for the appointment.
Trump chose Milley because he and Milley formed a close relationship during the interview process.”
Generals are world class butt kissers. No one can shmooze like a General or Admiral.