Because Mueller didn't indict Trump and some of the closest people around him in no way means the report will vindicate him or them. Mueller is very meticulous and circumspect. He has a good reason for not openly indicting Trump even if he has the evidence to do so. He could be leaving that to congress to decide what to do with that information, for other prosecutorial investigations (New York, D.C) to indict him later once he isn't president.
Mueller is smart, he knows Trump is going to try to exert executive privelage, that he will simply pardon his family. Russia waged an electronic campaign against us to sway our election. This has been proven. Hillary Clinton and other DNC emails were hacked, stolen, and made public. This is known.
Many members of Trumps family/administration for privy to this. This is known. Dr Jr admitted he went to the meeting for the documents, but claimed his dad knew nothing of it. Dozens of Trump's administrstion/family including himself have ties to Russia. That members of his administration (Flynn, Gates, Stone, Papadopoulos, Cohen, Jr, etc) had contact with Russia before and leading up to the 2016 election.
Muellers report will detail these many contacts. With testimony and other evidence compiled, the report will conclude that when all of this is put together, it creates a damning picture of Trump and his administration, and shows that he is far from innocent, and knew about Russia's war against our election, and in fact sanctioned it, by going after Russian owned dirt against Clinton, and not standing against it. That him and his administration openly worked with a foreign country to win the election.
Some fat guy in a bed indeed...that guy is Trump.
Not to mention all the obstruction of Justice.