I believe that someone once told Mueller "Don't be a hero", and he took it to heart.
The Twitterverse is of the opinion that the seven ongoing counterintelligence investigations have a lot of sticky bits that poke into Mueller territory and that public disclosure will have negative consequences. Of course, those counterintelligence investigations could simply be big rolled up balls of none-of-your-goddamn-business to keep Mueller from testifying.
He might have skeletons. His care in referring only to the report gives me the impression that he is sticking to a path that isn't his own. It's pretty clear that he doesn't want to express any opinion or interpretation that cannot be plainly found in the report.
He has a lot of work in the investigation, and I'm sure he would like to see something practical result from it. Given how politically difficult prosecuting a sitting president seems to be, making sure to do everything exactly by the book seems like a good tactic to avoid making a misstep that discredits the entire investigation.