The Democrats suffered a serious drop off in voters ~ Republicans failed to achieve as high a total as they had in 2004, but there was no 2008 to 2012 drop off.
At the same time there were still 10s of millions of potential Republican and Democrat voters out there who did not vote ~ as is usually the case.
We already know there's little mixing of the two bodies of voters ~ that sort of thing happens only over a long period of time and by the election, people who are going to vote know how they're going to vote. Probably the reason why someone who says he's switching is news ~ 'cause it doesn't happen all that often.
The purpose of a political party is, after all, to win elections. They do this by building up a following that can be depended on to vote for the party's candidates. To get the voters to turn out they present fresh, intelligent and ideologically compatible candidates.
One screw-up and you lose ~ e.g. you decide the party is really just a narrow faction of serious donors within the party and the others just go along, because, after all, where else can they go. Another one is to run old used up candidates with a reputation for being dull ~ or disinterested in issues where the voters are interested. Then, let's say you run a candidate who'd be better matched by the other party ~ well, let's just say what Harry Truman said, Democrat voters know that when faced with a choice between a real Democrat and a fake Democrat they'll pick the real Democrat every time.