Why is the A380's fuselage designed with a flat bottom?
There are 2 main reasons:

The wing spars run through that area. If you wanted to keep the wing spar within the oval cross-section, you'd have to install the wing much higher on the fuselage which means the wing spar goes through the passenger cabin.



Behind the wing spars, the landing gear is stored. The flat bottom provides more internal space.


Another image showing more detail of the structure: