When the start menu was left aligned, you can move you mouse infinitely to the lower left and still click it irrespective of the initial location of the mouse (There is a term for this concept in UX design called infinite space or similar). For similar reasons, the close (x) button is in the upper right corner.

However with the start menu in the center, you have to accurately place the mouse on the start icon and there cannot be a muscle memory since the movement depends on the initial location.