Hello there, captain alligator.
@Enzo has given you a little bit of an insight about exotic crocodilians in S.A zoos, but I'd like to dive more into that topic.
The answer is quite simple, although not for the exact same reason as Enzo pointed out. South America contains a large ammount of native crocodilian species (6 being caimans, 2 being crocodiles), so most of them are readily available for zoos to obtain (the exception being the critically endagered Orinoco crocodile), leading them to grab one or two species "to fill" the gap of a crocodilian on their collection.
Another reason is money. A majority of zoos in South America have little to no resources to even make reforms on their infrastructure, so importing exotic species is out of the table for most of them, and even if they do acquire the money to do so, exotic crocodilians would be far from a priority, due to the reason I pointed out above. This is why Buin is the zoo with the most ammount of exotic crocodilians (although I highly suspect they no longer have Nile crocodiles), because the zoo has the money to do so.
And last, but not least, I'll speak on the behalf of Brazilian zoos, since I'm not very familiar with those in other countries in the continent. No zoo or zoological institution in Brazil can import any kind of exotic crocodilians. Well, technically they can, but the political procedures to do so are very hard to be done, due to strict limmitations imposed by the government's environment organ. With this, most of the zoo's in the country gave up on keeping exotic crocodilians, since caimans are readily available and other exotic animals are much more appealing for those who can afford them.
The last zoos in Brazil with exotic crocodilians were the following:
-
Belo Horizonte's Zoobotanical Foundation: Belo Horizonte kept a single male crocodile called Irk, which certainly is one, if not the most misterious animal ever kept in a Brazilian zoo. Although this individual was at the zoo since it opened in 1959, there is little to no data bout him and its origins, which led the zoo's crew to sign him as a Nile crocodile until his death in 2014, but after recently analizing its skeleton and overall anatomy with a herpetologist friend of mine, we're pretty certain that he was a misslabeled American crocodile. He was also the last crocodile in the entirety of Brazil.
-
São Paulo Zoo: As pointed out by Enzo, São Paulo imported a pair of Tomistomas/False gharials in 1970, called Dentuço and Malaia, which were acquired before the importation limmitation was created. Dentuço, the male, actually breaked a record for being the heaviest tomistoma in captivity at around 700 kg, but his same weight would latter be the reason for his death, as he drowned inside his exhibit's pool. The female died some years later, in the late 2000's-early 2010's. As I'm aware of, they didn't have any offspring.
São Paulo did have other crocodilians as well, as there is a register on CITES of them importing two Mecistops during the 60's, and there is an image of an unknown young crocodile at the zoo of which we don't know anything about, but I strongly believe it was an American crocodile.
