Hello, Bill, I guess by writing "S970SX" you meant SX900 (or I missed a new one?);
I think the reason is quite simple. Before SX900 we had 12 buttons with style categories, now we have 10. All for the sake of a bigger screen. I can speculate that Yamaha didn't really want to use a touchscreen for PSR because they knew that it's not that practical, and it was the pressure of the modern culture when touchscreen is just new normal. Yamaha didn't really change the interface that much and it didn't make operating the instrument faster, in some cases it even slowed it down.