Trump recently publicly asked on Twitter why didn't Obama react sooner to the "Russian Hacking", since it all occured under Obama's administration. This is an excellent question, and actually such a meaningful and relevant one, that the MSM and Left immediately went into total overdrive with hysterical accusations against Trump shortly afterwards.
When the flak is heavy, you are over the target.
Consider two cases:
A RED PAST
There are three easy reasons why Obama would not go "nuclear" on a Russian media war against Hillary:
1) Vernon Jarret
2) Frank Marshall Davis
3) John Bachtell
These three men are all sworn Communists, and the FBI has files on all of them. These three men are only just a small sample of the Communists that mentored and shaped Obama.
You may ask, "So what? He had Communist mentors. What's the big deal?" Well, here's the problem: these men probably knew of other people who very likely did some very bad things in the US. In particular, Black Nationalists have always been infiltrated by Cuban intelligence and sometimes by the KGB itself. It isn't pretty.
If Obama were to truly go nuclear against Putin in a media war, what are the odds the Russians have an old document indicting a close friend or associate of Obama's with some heinous crime?
If the Russians incriminated Obama or his associates, publicly, in some horrible crime or treachery, then it would force the US government to respond: this is far more messy than letting Crooked Hillary lose the election.
THE ISRAEL & IRAN LEGACY
It is very important to understand the relationship Israel and Iran have with each other, because Israel and Iran are the reason why Obama has had such a weak position with Russia.
The Syria deal and Iran deal are really about one thing: peace in Israel. Israel has towns in the West Bank, a region that is not technically part of Israel yet is under Israeli military authority. Within the West Bank, after the Oslo Accord in 1993, are independent Palestinian Territories that are not under Israeli government nor military control.
It is within these Palestinian territories that Iran finances and orchestrates terrorist activity against Israel from the non-Israeli controlled regions within the West Bank. This means that if you want peace in Israel, you must negotiate with Iran.
Now, ignore Obama's long-ago past and consider his recent history: he publicly declared his intention to achieve a "two-state solution" in Israel, which means the end of many Israeli towns in the West Bank and the establishment of a much bigger Palestinian-controlled region in that area. Naturally, many Israelis, specifically those on the Right, were furious about this plan.
Obama's plan was not just a plan to "make peace" but also destroy the Israeli Right: by removing the "settlements" from the West Bank, he was redistricting on a massive scale. Imagine if we cut out those pesky red states that urban liberals complain about nonstop, today: that's what the Israeli Left and Obama sought to do within Israel.
This plan would mean the end of the West Bank dream and the complete collapse of the Israeli Right and the absolute victory of the Israeli Left. In this situation, losing an election literally meant you were evicted from your home months later!
As one can see, these are two very, very good reasons for Obama to keep quiet during the hacking against Hillary.