I think the implicit reason (which I too wish the article would state) is that if you (1) have bowed-out wheels for the front, and (2) want the benefit of all w

I think the implicit reason (which I too wish the article would state) is that if you (1) have bowed-out wheels for the front, and (2) want the benefit of all wheels being the same (easy replacement, etc), and also (3) want more wheels on the back (because that's where the load is), then the arrangement of bowed-out-face bolted to bowed-out-face solves it. If you had a bowed-out-face on the rear wheels, you would need some comically long bolts to join it with the other (inner) wheel.