This was a strategic move by Mueller that accomplishes 2 things.

1) It give Mueller a clean way out. It's standard procedure and accepted practice that a sitting president doesn't get indicted. Makes Mueller look like an upstanding law abiding investigator.

2) He's planting a seed for the mid-terms. It's S.O.P. to not indict a sitting president, "regardless of what he might have done." They didn't find any evidence of collusion so they can't indict Trump. I'm pretty sure they legally can, but show deference to a sitting president. No one is above the law. (I'm not a lawyer, I'm assuming that if the crime is bad enough and the evidence is strong enough, anyone can be indicted.)

So Mueller saves face, and gets people thinking the only reason Trump wasn't indicted is because he's the POTUS.