to point to a federal statute that was broken - beyond a reasonable doubt. Apparently, there is no law making it illegal for a political campaign to encourage and accept the help of a foreign adversary as long as they didn't actually engage in the conspiracy or illegal hacking with them. That's probably the crux of it when you read the report. The hackers hacked the email regardless of what Trump campaign members eventually did. They all knew what they were doing was wrong, but you probably can't pin down a federal statute that was violated if they didn't actively participate in the crime. Also, its apparently not a crime to not call the FBI and tell them that foreign agents just contacted you offering you "dirt" on your opponent. This comes down to the fact that no honest, legitimate candidate for POTUS would have encouraged the crime or welcome the benefits of such a crime, but the problem is Trump isn't an honest, legitimate candidate and the Constitution and Federal Code are woefully inadequate in addressing such issues because... well, nobody would have ever expected a candidate to act this way.
My guess as to what constrained Mueller in this. "Receiving a thing of value" which would be a campaign finance violation was probably the closest they could get, but then trying to prove the "thing of value" is the difficult part(under the law).