I think it's probably just that if you wanted a general-purpose large-volume cargo aircraft, this isn't what you'd design. They wanted a thing that fit very spe

I think it's probably just that if you wanted a general-purpose large-volume cargo aircraft, this isn't what you'd design. They wanted a thing that fit very specific, odd dimensions for a particular aircraft wing, and also didn't need very many of them, so they wanted to graft this all onto an off-the-shelf airframe. It ends up having what's probably an aerodynamically suboptimal shape, but Airbus doesn't care because at their low volumes, taking the hit there is easier than designing a new aircraft from the ground up that they'll only ever need a few of. Elsewhere in the industry, this particular shape probably isn't common, and if it were, there would be demand to design something better.