Beluga XL has cargo space volume 2209 m^3, while An-225 Mriya has 1300 m^3 (pressurized), cargo weight is 53 and 189 tonnes, respectively. So you need a big, "light" and non-pressure requiring cargo to justify the use of Beluga XL, which is exactly what Airbus is using it for, by transporting fuselages of other planes.
Slightly off topic, but for those interested in aircraft designed specifically for logistics (esp. Military Materiel), see the Caspian Sea Monster. It was meant to leverage the "Pelican Effect" of flying low over water with a wide wingspan; it was never meant to fly more than a few feet high.
These kinds of aircraft are built for special missions. This is not the first of it's kind. It reminds me of Super Guppy [1]. They are not very efficient aerodynamically due to the large frontal area. In addition, these are designed to carry oversized but not necessarily heavy loads; for example, the fuselage of another aircraft. So there is no point in mass producing them.
> This is not the first of it's kind. It reminds me of Super Guppy [1].
The predecessor to the Beluga XL was the Beluga (in the early 90s), and before that Airbus used Super Guppies. They bought two, then bought the right to build them and built two more.
Their point is to have enormous unpressurised volume ("oversized cargo"), their lifting capacity is not impressive compared to traditional cargo planes, especially for the price: each XL costs 200m (1bn for the entire program) for 50t, an An-224 can lift more than twice the weight for half the price.
Costs of operation. Airbus considered then abandoned the 380 Freighter, because despite the higher payload and range its higher operating costs than 747s (the reference for commercial cargo) were considered a non-starter.
While it was refitted and is active again, the An-225 was a strategic plane with the singular purpose of transporting Buran. After he dissolution of the USSR, it was mothballed for nearly a decade.
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.
