> Why is there only one amphibian?

Because amphibians are essentially made up of frogs/toads, salamanders, and caecilian.

People like frogs, it is an important emoji to have.

Salamanders on a small scale look too similar to a lizard.

Caecilians on a small scale look too similar to worms or snakes.

So the only reasonable amphibian to have an emoji is a frog.