Part of it can be attributed to sexuality. Most of the people described as the top influential of the last century have been described as 'oversexed' with the rest being made up of those who were either abstinent or impotent. JFK and and Churchill are prime examples of those on the promiscous side of the spectrum.
That isn't all there is to charisma, though, and having a high charisma can be learned.
The key is to overcome fear.