Why do primitive savages abandon their traditional beliefs for Christianity or Islam?

There is no more evidence for Christianity or Islam than there is for their traditional beliefs, yet so many savages have completely abandoned the beliefs and customs of their ancestors. Sure, some of them are forced to, but not all; many make the choice voluntarily to believe Jesus or Muhammad nonsense. Why is that? Obviously evidence isn't their primary truth verification process, so what is it that makes these people choose a new religion?