Acting harshly and aggressively towards Russian's actions during the run up to the election would have had significant implications and ramifications on his parties nominee who, at the time, was thought to be a near sure thing, hovering around 78% from august until election day. It would infuriate her opponents base, play into his accusations of the election not being fair ("wag the dog"), and likely tarnish whatever victory she obtained. It would also further strain the ability for the country to heal and come together behind her after her victory. What's more, assuming a Hillary victory, the political damage of aggressively coming out about Russia and their interference could be avoided with the assumption that President Clinton would deal with it post haste upon ascending to the office. Plus it helped maintain Obama's legacy, as entering into a significant conflict with Russia months before he left office would potentially tarnish his overall Presidential record in the history books (passing on his "problems" to another president ALA Bush and the WOT).
So Obama sought to engage in diplomatic negotiation strategies, minor diplomatic sanctions, and a relative downplaying of the whole thing in terms of any public speaking or focus on it so as not to damage his parties political opportunities, the countries ability to rally behind Hillary and heal after her victory, and the assumption that Hillary would win and then deal aggressively with the problem. So not so much "ignoring" it as much as kicking the can down the road with the heavy assumption that your partner is going to be on your 6 ready to kick it.
However...
The highly unexpected result occurred, Trump won, all the Russia stuff came out en masse actually causing the exact issue of the country having a hard time to heal to happen, and Trump...being the thin skinned individual he is...not strongly going after Russia like Hillary likely would because in his head, based on the presentation by the media he pays attention to, action against Russia is confirmation that they "stole" the election for him.