Their terrestrial life style combined with the need to return to the water to lay eggs hatching to larvae (tadpoles) led to a drive to abandon the larval stage and aquatic eggs.
A possible reason may have been competition for breeding ponds, to exploit drier environments with less access to open water, or to avoid predation on tadpoles by fish, a problem still plaguing modern amphibians.
Whatever the reason, the drive led to internal fertilization and direct development (completing the tadpole stage within the egg).
A striking parallel can be seen in the frog family Leptodactylidae, which has a very diverse reproductive system, including foam nests, non-feeding terrestrial tadpoles and direct development.
The Diadectomorphans generally being large animals would have had correspondingly large eggs, unable to survive on land.

Fully terrestrial life was achieved with the development of the amniote egg, where a number of membranous sacks protect the embryo and facilitate gas exchange between the egg and the atmosphere.
The first to evolve was probably the allantois, a sack that develops from the gut/yolk-sack.
This sack contains the embryo's nitrogenous waste (urea) during development, stopping it from poisoning the embryo.
A very small allantois is found in modern amphibians.
Later came the amnion surrounding the fetus proper, and the chorion, encompassing the amnion, allantois, and yolk-sack.
Their terrestrial life style combined with the need to return to the water to lay eggs hatching to larvae (tadpoles) led to a drive to abandon the larval stage and aquatic eggs. A possible reason may have been competition for breeding ponds, to exploit drier environments with less access to open water, or to avoid predation on tadpoles by fish, a problem still plaguing modern amphibians. Whatever the reason, the drive led to internal fertilization and direct development (completing the tadpole stage within the egg). A striking parallel can be seen in the frog family Leptodactylidae, which has a very diverse reproductive system, including foam nests, non-feeding terrestrial tadpoles and direct development. The Diadectomorphans generally being large animals would have had correspondingly large eggs, unable to survive on land.
Fully terrestrial life was achieved with the development of the amniote egg, where a number of membranous sacks protect the embryo and facilitate gas exchange between the egg and the atmosphere. The first to evolve was probably the allantois, a sack that develops from the gut/yolk-sack. This sack contains the embryo's nitrogenous waste (urea) during development, stopping it from poisoning the embryo. A very small allantois is found in modern amphibians. Later came the amnion surrounding the fetus proper, and the chorion, encompassing the amnion, allantois, and yolk-sack.
Reptiliomorpha