Why does my jelly crystal box single out pineapple, kiwi fruit and paw paw as preventing setting?
Certain enzymes (proteases) cut the protein bonds that create the mesh that causes the jelly (or Jello, or gelatin) to, well, gel.

Orange, watermelon and plum do not contain enough of those enzymes to interfere with gelling. In addition to paw paw (more commonly known as papaya in the US), pineapple and kiwi; mango, ginger, figs and guava also contain enough of those enzymes to interfere with gelling.

The application of high heat will inactivate those enzymes to the point that they will no longer interfere with gelling. That's why you can use canned pineapple, but not fresh, in gelatin desserts.