Because furtivism, habitat loss, contamination, sixth extinction, education and conservation.
Because some species only survive in zoos today.
Because only 9 of each 10 wild lions in Africa had dissapeared in one human generation lapse time, etc, etc...
Now they see that they can not properly take care of animals, even if they try to do their best they still do not know how to do it right. New sickness, etc.
The only way to save animals is by decree a land where human do not have any rights to go their.
The only way to save animals... [set them free, blah, blah, blah]
Then why the white rhinos are still endangered? Will survive the californian porpoise four years more? I, and many other people, seriously doubt it.
The whales and dolphins are not happy to do the show. This is proven and give them drugs (http://www.dauphinlibre.be/le-zoo-de-nuremberg-drogue-ses-da...) to kept them happy. Nice professionalism there.
Many other species lack diversity and therefore suffer from genetic diversity, they need to exchange species between country in order to grow the diversity, new sickness are born but yes they are professionals.
There's not real reason to stay in the old menagerie system, animals have rights to live and if we need food we need to protect them. Point barre, like we say in French.
Any real expert in dolphins can tell you that dolphins can not be safely sedated because, unlike us, the breath in cetaceans is voluntary. Basic zoology principles. Yes, zoologists and vets are professionals for something. They studied a lot of years and know their stuff. Surgery in dolphins is always a risk. If you put a dolphin to sleep, the dolphin "forgets to breath" and dies quickly.
Therefore the author of the article wrote some very unlikely accusations without any clear proof (and obviously do not know much about real dolphins). The idea of sedating dolphins regularly in a show for any extended period of time just because they "don't feel like dancing today" is ludicrous. This kind of defamatory articles are sadly as common as successful collecting money from their naive public.
I don't understand any french so I can't say much about that article, but as a local from Nuremberg, I can support the prior statement about them using drugs to keep their animals in check. I'm also not so sure about your statement because I found a paper regarding the effects of diazepam use on captive bottlenose dolphins [0], explicitly stating that:
"It is very tempting to use these medications as a management tool. They can be used to help mask the problems of poor husbandry, and of inappropriate and depauperate environments."
and:
"There is considerable potential for diazepam to be misused and the welfare of animals compromised. It may be used to help cover up serious deficiencies in management and husbandry, and there is strong evidence of it being misused in dolphinaria."
So this practice most certainly ain't as unheard of as you are implying.
They use psychotropic drugs, along with a rather long list of 20 other medicaments to keep the animals alive in those sorry living conditions [1], the data on the medication came straight out of the zoo's files. The zoo's director himself argued that giving dolphins diazepam is a totally normal thing to do to increase their appetite [2].
Even as a kid I never liked that place, seeing these animals perform tricks in a sterile indoor pool, with their back fins hanging down, felt unnatural and made me feel sorry for these poor creatures. They also had a hippo there that was housed in a tank that's barely been bigger than the hippo itself. I'm not an animal expert, but I have empathy and that was and still is enough to recognize what's happening there ain't good or healthy for these animals.
[0] http://endcap.eu/wp-content/uploads/2015/06/Diazepam-its-use...
[1] http://uk.whales.org/news/2012/07/german-zoo-data-reveals-ca...
[2] http://www.sueddeutsche.de/bayern/tierschuetzer-kritisieren-...
But what's described in the press briefing is already bad enough. On page 4 and 5 they list examples where dolphins got treated with Diazepam and even males with synthetic progestogen (female hormones) to influence their anti-social behavior resulting from the poor living conditions They even administered Diazepam to pregnant dolphins, the calf needed to be treated with antibiotics regularly because the mother would keep hurting it and in turn, the mother would get Diazepam in an attempt to tone her behavior down.
Because the anti-social behavior is so common dolphins regularly have to be isolated, furthering their mental issues.
Tbh none of this is surprising to me, in nature these animals are known to make 4.000 km journeys, 90 km a day, diving down to 500 meters. In Nuremberg 9 of them are stuck in shallow concrete pools with the total area of 1/4th of a soccer field. No room to evade social conflicts, nothing exciting to discover so, of course, these very intelligent animals are going crazy in such an environment, I know I would.
[0] http://de.whales.org/sites/default/files/pressebriefing_nuer...
And of course "Is very tempting to use it", "There is considerable potential for diazepam to be misused" and "It may be used for evil things" is not the same thing as "It was used for evil things". Almost anything could be harmful. There is considerable potential for coffe to be used for evil things also, but this is not its common use.
If, as the author claims, "there is strong evidence of their misuse in dolphinariums" please, show us the evidence.
In your third link in deustch says that the Berlin Zoo director Dag Encke claimed the accusations of drugged dolphins as being "bullshit" and "defamation" and that of course any ill dolphin is treated instead to let it die; an idea that seems sensible and do not supposes any moral problem to me.
Keep in mind that this is the very same director (he's director of the Nuremberg Zoo, Berlin doesn't have any dolphins) who demanded a dolphin breeding program gets started in Nuremberg to attract more visitors, even tho nobody on the staff had any experience with something like that. [0]
The very same director who thinks staff shouldn't intervene when dolphins attack each other because "that wouldn't happen in nature" [0], never mind that in nature these animals have quite a bit more room to evade each other than a somewhat bigger swimming pool.
[0] http://www.zeit.de/2015/04/delfine-tierschutz-delfinarium-nu...
Think of zoos as the world's DR environment.