there are many possible answers to either your question or mine. In fact one of those answers could be that they did in fact do this, but it is not being reported by your (or my) biased news sources.
But I will play..........if your assertion is true.....
...my guess is that Trump knows that he didn't collude with Russia, so he thinks it is all B.S. so why waste time on it? The idea that $100,000 spent of Facebook ads in the week leading up to the election would sway an election and convince people who were Hillary voters to now vote for Trump is ludicrous also.
....also, it is my guess that Obama never thought Hillary was going to lose, and didn't feel any pressing need to address what he thought was a minor issue (or he knew there was serious dirt in the emails, and he wanted to let sleeping dogs lie until after the election, when all of his corrupt DOJ and FBI heads (the suits, not the boots) could cover the tracks.