Because they don't like the EU and think somehow the UK will have still some clout in international affairs if we were on our own.

It should be noted that the EU is not just concerned with the ivory trade but illegal trade in other wildlife products both within and without the EU.

There is concern that elephants are evolving themselves out of the ivory buisiness by producing young that don't have tusks though that puts them at a disadvantage when for example, grubbing for food.