16:9 took off because most users appreciated the difference between it and 4:3. Philips have been pushing 21:9 for ages, the demand is not there. 16:9 is close to the golden ratio, the ratio the Greeks found to be most pleasing and used in their architecture. I think someone calculad is the natural aspect of the view from two eye side by side. When you go to a movie thwter you are basically your own pan/scan system, extracting your own 16:9 view from the huge image that you can't take in from a single viewpoint.
Some things pushed on us are plain daft. Unless you have a huge curved screen, this sitting off centre will start to see more of the edge than the screen. It's as if someone said we can make curved screens, and no one said "so what?" . And they wonder why the TV industry is in difficulties.