Obama to Wait Until Senate Is in Session For SCOTUS Nomination
It's possible that some candidates on the short list this time were also seriously considered previously, so they had already been partially vetted. But in the end, it's probable that the president decided not to nominate a half-vetted candidate in order to avoid any trouble.
Not that it really matters. There is not going to be a groundswell of public pressure to hold hearings and vote to confirm a new Supreme Court justice. The inside-the-beltway pundits might get their panties in a twist over GOP "obstructionism," but the issue will not resonate with the voters. The Supreme Court has functioned just fine in the past with eight members and it will do so again this time.