Why is the lower deck of the Airbus A380 curved like a single deck airplane?
The cabin interior is curved to accommodate, among other things, the brackets holding the upper deck floor beams.

These brackets are a structural feature that transfers the weight of the upper deck to the fuselage with considerably less reinforcement that a straight T-joint. The bracket acts as a small truss element, creating a triangle with the deck and the fuselage that partially turns bending loads into axial ones.

Here is an actual production cutaway, noticeably different from wiki's simplified representation of the A380's cross-section:



It would be possible to only wrap the interior lining around the brackets themselves, not along the whole length. But they're not the only thing there (although others could be rerouted; structure requires testing), and this curve saves a bit of weight, so Airbus curves the whole interior by default. An airline could request something different if they wanted to.