AAA has given the correct answer: Obama was lame duck, whereas Mitch is entitled to presume that his party's president will be re-elected even if it does not happen.
Now that AAA has given the correct answer, I will add to it by giving a politically incorrect answer: ie. don't even fucking talk to me about the morality of this or that or the various procedural civilities and expectations that should prevail AFTER WHAT WAS DONE TO KAVANAUGH. If Trump loses the election and a justice dies the next day, I would do everthing possible to ram another justice in in the last couple months even if Trump loses. I would extract the absolute total possible retribution for what they did to Kavanaugh. And dont anyone even bother to argue that it would set a precedent such that the dems will do this or that if they are in power. We know what the dems are going to do regardless. They will turn down no chance to knife either republicans or established processes or courtesies at anytime, anywhere so no one should be trying to earn protection.
Mitch started it....plus Kavanaugh wasn't entirely cleared