> But why?

To get the little-endian ordering. The place values of digits increase from left to right - in the same direction as how we write literature (assuming LTR scripts), allowing us to do arithmetic operations (addition, multiplication, etc) in the same direction.

> The brilliance of 8601/3339 is that string sorting is also correct datetime sorting.

I hadn't thought about that. But it does reveal something interesting. In literature, we assign the highest significance to the left-most (first) letter - in the direction opposite to how we write. This needs a bit more contemplation.