For one thing, because it was Barr that said that, not Mueller, and Barr was hired because he wrote a memo last year that essentially said that nothing Trump could do would be obstruction of justice.
Mueller, on the other hand, said "[T]he investigation did not establish that members of the Trump Campaign conspired or coordinated with the Russian government in its election interference activities"
That is a nothing like "No evidence of collusion". Furthermore, the report did find that Trump told the Russians in a public speech to go after Hillary's server, and 5 hours later they had attacked her computer. You can say that isn't "collusion", but it's sure an awfully big coincidence.