First a correction: The Trump investigation began in mid 2016 on the basis of the false Dossier.
Now, as to why this wasn't leaked to the press as everything else was after the election. Consider:
Obama himself has already been proven to have been tipped off as to the DNC hack, but did nothing to the Russians for months - in fact he only issued a reprimand and symbolic sanctions in the very end stage of his lame duck session. So why was that?
Answer: He didn't think it that unusual given the past history of both the US and Russia wrt each other's elections.
He knew there was heavy dirt on both Hillary and the Dems, and he didn't want it exposed.
He had not yet sen the drip/drip/drip techniques that Wikileaks used to roast Hillary.
He knew there was collusion between the Clinton's and Russia (and many other countries), and he didn't want that coming out. He never expected that Hillary would lose, and thus wanted to keep things under wraps if at all possible.
Now, AFTER the election, they had a meeting where it was decided to simply accuse Trump of their own sins. To this day, there is only evidence of Clinton/Russia collusion wrt The Dossier (where Clinton and the DNC paid Fusion GPS to in turn pay Russia to leak a fictional Dossier on Trump as "an insurance policy as mentioned in Strzok's email), and wrt the Uranium One deal which resulted in a huge cash donation to the Clinton Foundation in exchange for 20% of the US uranium supply.
Since the MSM is literally the Pravda mouthpiece of the democratic party, and because Obama did not want this information coming to light when he had no doubts about Hillary winning the election, he simply didn't instruct that the information be released by the complicit press. We already saw in the Wikileaks how major NYT writers ran their articles by John podesta for approval. No story like this would ever be written, much less published without the approval of the Dem Party first.
It was only after the election, that the Deep State leakers cared about leaking anything (anything that would hurt Trump), or the MSM cared anything about actually doing investigative reporting - well ok, they still don't really do that, they just make up Fake News - but you get my drift.