I believe a factor in this is the incentives at play based on a mentality.

The incentives are that if data is deleted, the person doing it can be in trouble for destroying it. The probability of getting in trouble for not deleting is low.

This is based off of a mentality of: If we need the data and delete it, we (the gov) will have less power. If we don't delete it and need it, we will have more.

I believe the flaw in this reasoning is not considering a 3rd possibility: We don't delete it, don't need it, but it falls into other (even less scrupulous hands) that can use it and gain power.