Bill Clinton probably has a strong fan following due to his accomplishments. Many people like to follow former presidents because they consider them to be similar to celebrities. These former presidents still go out in public and perform community service events. Some of them like Bill Clinton give speeches and many people go to see him.
Another thing that Bill Clinton did after his presidency is to help a group of three journalists escape from North Korea. He was able to negotiate with North Korea which was at odds with the United States at the time for the release of these three U.S. journalists. This made him look good and it also probably created a bigger fan following for the former president.