Because for a street rider, it's an easier and safer pattern and it follows logic.
From neutral, a quick, easy, stab down puts you in gear to take off. Lift up on the shifter to upshift, push down to downshift. Also, when stopping in an emergency, it's easier to make stabs down for downshifting than to get your foot under the shifter.
(I know, I had a bike that shifted in a 4-down pattern - neutral was easy to find, but panic downshifting was a PIA.)