Muellers job was not to exonerate the president. It was to find a crime if one existed and then charge or list the crimes for Congress to deal with. We have good examples of how this works from both the Nixon and Clinton investigations. In both cases, the crimes were listed and it was left to Congress to deal with it. People other than the president were charged and taken to court.
In this case, the only crimes having to do with "collusion" or election interference were listed in indictments brought against Russians. The other crimes were mostly process crimes and had nothing to do with collusion or Russian interference. The exception was Mannafort, and his crimes had nothing to do with Trump or the election.
In this country, as in your country, there is a presumption of innocence. If there is not enough evidence to prove a crime you are innocent. Mueller listed some hearsay stuff that Trump said, but there were no follow on actions, and intent can't be proved. Obstruction requires intent and action. One of Clintons crimes was obstruction and Starr showed how he had both intent and action.
Impeachment can be done absent the above proof. Even though it is a legal proceeding within Congress and has different rules, it is not a guilty or innocent thing because it requires 2/3rds of the Senate to finalize and remove from office. Impeachment can pass without the kind of legal proof that is required in court. In this case, there is no danger of the Senate finding 2/3 of the members to approve it and if Mueller could not find proof of malfeasance, the Congress will not.
There are at least two other investigations going on into how this all got started and what the FBI and CIA did. It is likely that the lawbreaking took place within the investigation. It seems the Brits might have helped.

One of the reasons Trump did not earlier declassify the info is that he was asked not to as it would embarrass or impact our relationships with foreign intel agencies. It's one of those things that ought to have caught the attention of the press, but did not.
There is far more to this story than you are reading in the MSM.