Laptops with touch screens started to become standard around that time. Simplifying the UI was targeting these devices. Ubuntu’s Unity was another DE that came out targeting touch.
I’m not saying I liked GNOME 3, or agreed with their decision to make life more difficult for mouse navigation and longtime power users, but it was easier to navigate with a finger. That was the obvious reason why they did it.
Heh. Laptops with touchscreens are really bad for hopefully obvious ergonomic reasons ("gorilla arm syndrome"). I've had such laptops and I never use the touchscreen except by accident. I thought it might be useful to test multi-touch interactions, but actually! you can multi-touch the touchpad and that does the same things as multitouch on a touchscreen.
I’m not saying I liked GNOME 3, or agreed with their decision to make life more difficult for mouse navigation and longtime power users, but it was easier to navigate with a finger. That was the obvious reason why they did it.