What were they expecting Mueller would find?!

The main perpetrator is Putin. It was Putin who instigated hatred and populism in Europe, Brexit in UK, and eventually helped a jingoistic fear monger be elected into office in the US, by leveraging his immense and effective propaganda machinery.

Trump welcomed this support, but it doesn't make Trump the perpetrator, he was merely picked by Putin as the favored candidate. Therefore, there was probably nothing incriminating about Trump in this affair that Mueller could hold on to.

It was Putin. Late Senator McCain even went on to say that the whole purpose of the Syria intervention was to create chaos and rout masses of refugees into Europe and thus instill fear and hatred, which probably isn't too far from the truth. And yet, no one is investigating Putin, because he's outside of the FBI's jurisdiction. However, Putin is able to exert influence well into the United States...