I think because 8 is not supposed to be boxed (because is always field). If it was in the center, then you had less posibilities.
I actually bought into this argument. Until I looked hard at it. Province 8 can assist you to box in provinces 7, 9, or 13 . . . but if province 13 was your starting point you could use it to assist in boxing in provinces 8,12,14, or 18 . . . 4 choices not three . . . the question is still valid. . why don't we start in province 13?