Hi OP, can you specify which geographic region / jurisdiction (e.g. country, city) and time period you're asking about? That will greatly assist anyone considering answering here. Thanks

Six feet is the traditional depth (I have not heard that it was ever eight feet deep). I have heard people - in the context of folklore research - suggest that it should be roughly as deep as the person is tall. Six feet covered most cases, according to this explanation.

That said, the folk often arrive at explanations for traditions they have inherited but do not understand (folklorists refer to aspects of folklore that are not clearly understood as "blind motifs"). There is possibly some reason for this to exist that is "grounded" in fact rather than folk speculation. But grave depth is a tradition, and even written sources on this are likely to speak with more speculation than authority.