Why are bryophytes called amphibians of plant kingdom?

Bryophytes, contrary to other plants can't live in a water free environnement.

Because they need water for reproduction. The Bryophytes are called amphibians of the plant world because they cannot live away from water. While reproducing, the gamete produced by the bryophyte will need water to travel from the male gametophyte to the femelle one. It works like flower's pollen in the wind but here in water.

As you can see, the gamete are release in a "goutte d'eau" witch is french for "drop of water". those drops full of gamete will then flow on the plants and fall on one the female gametophyte. Therefor water is an essential part of bryophyte life.