The A380's engines were developed in the early 2000's.

Newer twin-engine planes (B737-MAX, A320neo, A330neo, A350, B787, the upcoming 777-X) have improvements in engine efficiency that have not been ported over to the A380. Airlines operate on wafer-thin margins, so even a percentage point increase in engine efficiency is huge.

Granted, the engines themselves are not made by Airbus, but there are integration costs involved.

There are other economic issues: (1) It's just hard to fill an A380 year-round. Not an issue on trunk routes like SFO-LHR, but there are only so many of those. (2) If slots at airports were auctioned out, you'd arguably see more demand for larger planes - maybe not the A380, but 777 or 787 sized.