If you take a look at the size of widescreen monitors, you can kinda guess why someone decided to move the start button/menu to the middle of the screen.
I know Samsung and Dell have ginmorous 49 inch monitors. Start menu that pops up from the lower left corner of the monitor would be a bad UX - the user might not even notice that a menu had popped up if that lower left corner of the monitor is out of their peripheral vision.
Moving the Start menu to the middle of the screen does go against years of muscle memory... moving your mouse/trackpad to the lower left, using the monitor border as a stop-zone though.
I guess they didn't want to make it an option/toggle hidden in some dialog box somewhere...