Answer:
This is because predators will find the easiest meals to snack on, even if that includes hunting down endangered or vulnerable species they're supposed to protect.
Step-by-step explanation:
An example would be the Hawaiian mongoose case. If you don't know, mongoose are very popular due to their brave and aggressive nature, where they'll hunt pests down from rats eating on crop fields to even giant snakes in India that would other wise kill humans in a single venomous bite. This brought up the idea of bringing mongoose to Hawaii, since some rats were being a problem with their agriculture. So they brought 72 of them and started breeding them.
This immediately backfired as they caused many native birds and other native animals trouble as they were eating its eggs. There were so few mongoose hunting down the rats that it was rather better for them to not bring it in the first place. This is why today, Hawaii is very strict on what animal you choose to bring, even your own dog!
Another example would be the Australian dingoes. Except, this was unintentional. A few thousand years ago, their once-domesticated dog ancestors were actually abandoned by their owners from some Southeast Asian area and came down to Australia. They outcompeted existing predators such as the Tasmanian devils on Australia. They simply could produce and maintain more offspring than the Tasmanian devils. This caused the extinction of the last Tasmanian devils in the last couple of decades.
Although the dingoes aren't as bad as the mongoose, who ate everything they could find, as they hunted feral pigs as well as rare hunts on feral cats, they also eat the threatened native species they're meant to protect.
Last and very good example would be New Zealand, home to the most vulnerable birds: Kiwis and Kakapos. Although the taxonomy shows that they have individually evolved flightlessness, they both are vulnerable species. This is because, similar to the dodos, they didn't have predators to begin with. And you can't have a predator to attack the existing invasive species since they'll just snack up on the flightless birds as they're an easier snack anyways.
The more an ecosystem has been isolated, the harder it is to restore balance once invasive species have been introduced. Predators will not hesitate to try new food, such as kiwis. Think about it, if you walk into a pastry and see a new type of cake on display, you might as well give it a go. Predators are just too unpredictable that it's best to leave an ecosystem alone.