Jersey always seem to have a generic explanation for anything in their collection which is not endangered, which is that they are perfecting husbandry methods on a more common species before switching to a similar, rarer one. This has been stated with reference to coatis, short-clawed otters, and meerkats among others, but with the exception of the Narrow-striped mongoose, they never seem to acquire anything to replace these animals.
With the Howler monkeys, these are simply described as a 'model' species to represent endangered wild relatives. I think this is how the collection at Jersey will develop now, as the conservation work of the organisation gravitates further away from ex situ captive breeding, especially for mammals and larger birds, while the zoo site is required to generate revenue rather than deplete funds. I guess most people don't care about paying to see critically endangered reptiles and amphibians so much.