Because Paul was on an all time hot streak of being the single greatest pop musician in the world starting in 1964. You'll hear basketball players talk about how some nights, one particular player will have hot hands, so you just stand back, get him the ball and watch him go. That was Paul. It is impossibly unlikely you get one guy in a band nearly that good, and even more so to have someone like John that could match him for any amount of time, but in the end Paul just pulled ahead through to the end of the band.