The title says it all.
For much the same reason as Bruce Springsteen was called “The Boss” and Elvis Presley “The King”. he was thought at the time to be the best of the best.
We know better now, of course.
He really was the Chairman of the Board of Reprise Records, and the title stuck outside the biz.
Because all the other members of the “Rat Pack” had to kiss his ass. 
A guy I used to work with referred to Sinatra as “the Darth Vader of Swing.”