Sometimes, to understand what is happening, it helps to observe what is not happening. One hallmark of Trump's success is the limited use of the media's customary favorite election-year pejorative label - the "flip-flopper". Sure, there have been plenty of accusations, but not the drumbeat we have seen in the past. Like the word "lie", the accusation has a broad enough application to avoid intelligent scrutiny and be made to stick. Absolute proof of meaningful flip-flopping is elusive, leaving listeners to apply their own worldview. For John Kerry, who as a Senator used votes for and against bills with similar or identical provisions as a parliamentary tactic, the accusation was a made in a one-two punch using the doctored visual of his windsurfing. Turning a strength - his physical fitness-into a weakness - "flip-flopper" is standard playbook - making for double duty. If the "flip-flopper" is a long time favorite of Republican strategists and a media darling, why fewer flip-flopper accusation this year?
When the accusation is lying and the pejorative label of choice is "liar", you don't need "flip-flopping". "Liar" as a label means that you are always lying. "Flip-flopper" means you say one thing and then turn around and say the opposite. If a "flip-flopper" is lying, then turns around and says the opposite, that would be the truth, but if he or she is a "liar", that is impossible. If the candidate is being accused of lying and being accused of flip-flopping, both of which imply a permanent state, a contradiction on a particular issue could arise, calling the accuracy of both labels into question. The accuser does not want this.
The clearest instance of outright "flip-flopping" is Trump's reversal on the legitimacy of the Obama presidency based on his birthplace. Neither Democrats nor the media accuse him of "flip-flopping", the one-time favorite label to discredit a candidate, favored by Republicans, but used by both parties, because the substance of the Trump actions shows them to be so repugnant on their face, that there is no added value to the flip-flop label which thus becomes a distraction. That label is generally more about gamesmanship than substance - and so is the "liar" label by the way - that we do not see it used much this year.
Sam Stein at HuffPo has his own take Why the Clinton Campaign Isn't Attacking Trump As A Flip-Flopper