When the A380 has been developed, winglets were already well understood. Adding winglets does not only improve aerodynamics, it also adds weight. Weight saving was a big issue on the A380, as it is already the heaviest aircraft on the market, and compatibility with existing airports and the load ratings of their surfaces was important. By now Airbus has learned that not too many operators make use of the full range and hence the MTOW of the A380, there seems to be room for some structural weight increase now (also to shift some weight from fuel to additional seats). Additionally all structural testing (lasting some years beyond entry into service) has probably shown some margin for increased weight as well.
And of course there is always a marketing issue with winglets... If the operators do want them, the manufacturers offer them. Even if it is not the technically best solution.
It will of course not solve the issue that A380 passengers do experience on airports which are simply not capable of handling that number of passengers properly... The nicest and most comfortable A380 flight becomes a nightmare if you cue up an hour for immigration at your destination, and wait another 30 minutes for your luggage after that.