Airbus bet on a bigger airplane built with traditional techniques while Boeing bet on a smaller airplane made with new techniques.
Other than its size, the A380 is still surprisingly efficient despite engines that are largely outdated (compared to 787/A350 engines). But double deckers are very efficient by design, so maybe it's not that surprising. I think the only airliners more efficient currently flying (in terms of fuel per passenger, with normalized density/layout) are the A350-1000 and 787-10 (though one could argue the latter is more of a mid-range aircraft, so not directly comparable). The 777-9 will also beat it. Even more surprising when realizing that the A380 is in fact a shrink: there was supposed to be an even bigger (longer) version, so it's overbuilt (read: overweight) for its size.
But empty seats (or seats sold at very high discount, or a low seat count because the airline knows it's not going to sell that many tickets to begin with) don't make it economically viable on most routes out there. Besides Emirates only British Airways seems very happy with them. Even the 777-9 is starting to feel like it's too big, with very few airlines ordering it. Even the smaller A350-1000 is not raking in orders...
The A380 makes pretty extensive use of composites. It's only conservative in this respect compared to the Dreamliner.