How do these percentages explain why the Democrats adopted several Populist planks and nominated the Populist candidate as their own nominee in 1896?

Well, honey, let me break it down for you. The Democrats saw those juicy Populist percentages and thought, "Hey, we could use some of that popularity!" So, they snatched up those Populist planks and even nominated the Populist candidate as their own. It was all about riding that wave of public support, no matter where it came from.