When the subjects of such cases are small children, many of the normal explanations that could apply to adult cases are unlikely, if possible at all.
Small children have not learned anything from newspapers or novels. They cannot draw upon years of adult experience in the world. Their contacts with other persons and places are limited.
Fraud is a possibility, and may sometimes take place. But cases in which fraud seems ruled out, cases carefully investigated by scientists on the lookout for fraud, now number in the thousands.
When children speak of past lives, their parents often discourage them. And the parents, far from seeking to profit from the unusual circumstance, are sometimes reluctant for the case to become known.
Discrepancies in investigative technique? These are also possible. But merely alleging them is not enough. There are too many published cases in which the techniques and reporting appear meticulous.
Normal explanations may still be possible, of course, but they become harder to come up with and support.