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.
https://www.youtube.com/watch?v=bFkIG9S2Mmg - The Insane Biology of: The Axolotl
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.