re: How did it come to this?

The GOP had a majority split against Trump in the primaries. Until Cruz dropped out he hadn't won a single one in closed primaries. Crossover voters gave him the nomination. And establishment GOP hated Cruz so much, they were doing whatever they could to keep him from getting it, no matter if it meant Trump getting the nomination and then undermining him for Hillary. The GOP Establishment acquiesced to every policy and legislation that Obama wanted, while holding the majority. I think that's why you're seeing such a high turnout.