If your feet slip on platforms, you have cheap platforms. Switching from nylon Wally World flats to Kore Race II's, I had more overall grip than I ever did with clipless. Can't pull up with them, but definitely better lateral grip than the SPD's on my road bike. And I'm still riding with gripless skateboarding shoes (flat, non-waffle outsole). Still deciding between a pair of FiveTens or some new jeans to get me through the colder months.

To answer the question, my only assumption would be that clipless pedals were over-produced & flats are now making somewhat of a comback as the DH scene starts gaining mainstream fame outside the NW.