Inherited temperament. Same reason why some kids born to alcoholic parents become alcoholics while another child in the same environment avoids that pitfall.
If you are high on the psychological trait called "openness to experience" (google it) you will eventually question everything. If you are low on that trait you probably will be afraid of the discomfort of change. This quality is mildly correlated with intelligence. In fact if you are too intelligent you may dig a deep intellectual hole of "belief" and become a fanatic.
This is why it's silly to condemn people for their choices. Contrary to religous teaching our choices really aren't "free". Our inheirted disposition trumps most environmental effects.
That's why its so important to "know thyself".