The amniotic egg was a successful adaptation primarily because

The egg of the amniotes are surrounded by the set of the fluid membranes known as amnion. The fluid present inside the amnion is known as amniotic fluid.

The evolution of the amniotic eggs removes the danger of the desiccation of the eggs. This fluid provide the nutrient to eggs and protect the embryo from the physical danger. The egg shell of the amnion allow the transfer of the gases in the developing embryo.