You can't get a conviction on a charge like that without clear proof and the prosecutors know this - which is why it went the way it did.
This is debatable, but there are a few reasons they did not go that way.
First, it keeps DOJ apolitical, because if convicted, that would disqualify him from holding office.
Second, that charge has not happened for over 100 years - meaning yes it would be a hard get. They went for more favorable charges and that's exactly what good prosecutors do.
The DOJ is not looking to bury Trump politically as you might think. They just want him to have his comeuppance for what he actually did, whether that's J6 or the documents case.
74% of Republicans think that Trump as a dictator would be good for America,
I kinda feel these people wouldn't care if Trump was President until he died.
They live in their bubble and just want closed borders, access to their guns, and a leader who talks smack.