Emirates has just agreed to buy 20 more A380s (with an option for 16), but very few other airlines are interested because they carry too many passengers. That makes them very inflexible for an airline on which routes they can fly, and be profitable. To be profitable, you have to fill almost all the seats, every flight, all year round. There are only so many routes that can do that, and Emirates have already sucked a lot of those passengers up on their existing routes, flying through their Dubai hub. There aren't a lot more passengers to attract, so they do not need more A380s.
"The A380 is not made for every route, but it is ideal for high-traffic routes, high-volume routes that are congested or where there are flying constraints,"
(From "Bigger isn't always better: Why the A380 super jumbo is struggling to take-off").
And, if there is a slight downturn in passenger numbers, there's not much you can do with an A380, but fly it a bit empty. However, if you have smaller aircraft, you can run two medium aircraft when the demand is heavy, or one medium and one small one when the demand is a bit less, or just one small one, in the off-season. But... you can't do that with an A380. You either go 3/4 full, or not at all. So, it's inflexible for route planning.
Qantas originally order 20 of the double deckers but decided to only take delivery of 12. According to Joyce, the Dreamliner offers the airline more flexibility and lower financial risk especially on routes with inconsistent or seasonal demand.
“If we were to fly two (236-seat) 787s tail-to-tail, the per-seat cost would be less than the (486-seat) A380,” Qantas CEO Alan Joyce said in an interview.
(From "We may have just witnessed the end of the A380 super jumbo").
Airlines don't buy aircraft just because they have the latest technology. Airlines buy aircraft that will have a lower cost per seat-mile. This should be an aircraft with the latest technology, but, if that technology comes in an aircraft with more seats than you can fill, then the cost per seat-mile isn't as good as it should be, and may be higher than a smaller, older aircraft.
So, simply, the A380 is too big. It makes an airline put all their eggs in one basket, and only Emirates had the routes and traffic to buy it in any quantity.