Trump didn't collude to get elected
We all know that Trump has admitted he never really wanted nor expected to actually win. He was in it for fame, and to get rich. He wanted to hurt Hillary as a way to gain stature, and therefore reputation, which he would turn into financial opportunities. That is all his brain knew or could really handle. It just so happens that Russia really did want to sway the results of the election. So, you put those two things together, and you get what we got.
Mueller's investigation was narrow (we said/heard this nearly daily): did the Trump campaign collude with Russia *to influence the results of the election*. In this regard, Mueller found no evidence of that specific thing.
What Barr is burying, however, is all the "mundane" criminal activity that Mueller found that was outside of his mandate. If it wasn't election collusion or obstruction, he had to refer it all. All of it. Even Trump's and his family's criminal acts.
This is why Trump was nervous the whole time - not because of collusion (which he knew he either wasn't guilty of, or knew the Feds didn't have the info about it due to all the leaks), but because of all the crime they would find. We'll see Barr and Trump fight hard to suppress all those parts of the report, arguing they are sensitive or secret. I am sure the Dems know all of this, and they will not stop until they get the unredacted report, even if its not publicly available to us.