'collusion' isn't really a crime, nor a thing.
they would have had to have found clear evidence of treason or campaign finance violations, which it would appear he didn't. I don't know that it's fair to even really say he 'punted'. I think he gathered all the facts, there wasn't a good case for treason nor campaign finance violations, so there were no charges to be brought up nor any reason to ask congress to impeach.
all the mueller report really proved is that Trump
tried to obstruct justice a couple times, but people wouldn't follow through, so it never happened.
I'm not for prosecuting people for crimes they wanted to commit...
Hell, I'd be serving 90 life sentences right now.