Why didn't Mueller charge people other than Trump with Obstruction Of Justice for deleting data?

[pnwmom]

2. How would we prove it?

In some cases, at least, the reason we would know it is because another country's intelligence service gave us data that they could collect but we were legally barred from collecting ourselves. Such as if one of our citizens traveled abroad and another country spied on him.

Having that spy data we would know that the target withheld it from us. But we can't take the other intelligence services' data to court without betraying sources and methods. So to keep the lines of communication open between our country and others, we don't bring it to court, even if it means we can't always bring a case. Having the intelligence is more important in most cases.

With regard to apps with encryption or make conversations disappear, if it is legal we can't charge someone for using it. And if it works we don't have the data.