There are proportionally more fanatics than rational persons, it would seem, and two reasons are that most persons do not have an intellect that is good enough to set up their own philosophy, while being a fanatic is intellectually open to all and  tends to be emotionally satisfying and also may come with great if delusive promises of happiness in the after-life or once the millenium has been created, or with rewards for those who fanatically serve a leader in this life.

It should also be mentioned that in some situations it takes great courage or independence of mind not to become - or at least to behave as - a fanatic, for example, under Stalinism, Maoism, in communist North-Korea, or in territories were people are much repressed or persecuted.

Also, two excuses of many fanatics - which they will tend not accept themselves - is that their social environment is totalitarian, and that their intelligence is not very great.