Right. Remember that part of the design mission is zero carbon emissions.
That means LH2 (liquid hydrogen) fuel. Which is only liquid at extremely low temperatures and/or extremely high pressures (something like 500 bars/atmospheres or 50 million pascals). It ain't kerosene (or even Zip fuel, which is just kerosene with boranes added).
So you need a substantial
pressure vessel for the fuel. You can't just tuck it into nooks and crannies throughout the aircraft (and certainly not the leaky wings of the SR-71
).
H2 is not very dense, even in liquid form (~70 Kg per cubic meter) - but it is bulky. Thus the fat fuselage that is 50% pressure tank.
The Airbus design makes a virtue out of necessity by putting the delta wing on top, and giving the fuselage around the tank a wedge shape to promote compression lift below the wing (which the XB-70 also did, with the aft fuselage).
Think of this Airbus proposal as the Space Shuttle - with the orbiter cabin placed on the nose of the fuel tank instead of on top, and within one aerodynamic skin. Or as a fatter XB-70, with the "crank" in the fuselage/engine box straightened out.
The other design goal is
minimum sonic impact on populated areas. Neither the SR-71 or the XB-70 could
accelerate through Mach 1 (and on up to Mach 4.5)
while in a vertical climb (70°+ pitch angle) at or above 45,000 feet. Not even close.
Concorde could reach Mach 2 only with a long, slow climb of about 20 minutes between ~27,000 feet and 51,000 feet, and a pitch of 4-5 degrees. The SR-71 had to level off at 33,000 feet and enter a
descent (the "dipsy-doodle") to punch through Mach 1, followed by a Concorde-like climb/accleration at a constant 450 KEAS.