I think depending on your viewpoint, this could be considered in two ways.
1. It was clear and obvious the payoff and coverup was meant to deceive Mr Trump's wife and was thus a personal affair and hence not criminal.
2. The parties involved realized the two stories would negatively impact a campaign which they did not think they could win. hmmm ... if they didn't think they could win why would they care what the impact would be?
Trump has apparently tripped to this reality: President Trump defends payments to women as 'private transaction'
(USA Today). The problem is twofold: 1) it's a lie ('natch), and 2) timing. If it was personal, it could have been taken care of in the decade it occurred. They were made only after the election possibility was real.