Because the lungs of amphibians do not provide them with oxygen in full, then plays a big role in skin breath, the wet thin skin. Therefore, amphibians can live only in a fairly damp places, moreover, only in fresh water.
Amphibians are very demanding to the absence in the soil and the water table salt, which is particularly harmful effect on the development of their eggs and larvae. They die in water containing 1 part of sodium chloride per 1 thousand parts of water. The reason for this is that the salt concentration in the cells of the skin of amphibians is less than in seawater. And if amphibian will fall into the sea water, water from the skin cells will freely go outside, because their skin is very permeable to water. And the animal will die.
That is why no amphibians on oceanic Islands, while the rest of the conditions there are favorable.