The reason is capslock is on the home row, and control is not. I use a Kinesis Advantage keyboard, so ctrl is also in a good spot (thumb), but for most people, less stress would be to use the homerow key rather than bend their pinky to hit ctrl. It's a minor thing but it adds up.

If you use escape with vim you're actually "doing it wrong" in the sense that vim was not designed for your keyboard. On older keyboards escape was where the Tab key is now. There's no reason to abuse your wrists with repetitive motions going up to the escape key.