As we know that frogs and salamander are amphibians but why do frogs reproduces externally and salamander reproduces internally?

Reproduction in amphibians: Frog and Salamander:

• Amphibians usually reproduce via sexual reproduction.
• Generally, amphibians lay large numbers of eggs at one time, and they often do so in groups, all depositing their eggs in the same place at the same time for external fertilization.
• They are tied to an aquatic environment for reproduction which go through a larval stage that is very different from the adult form.
• Fertilization is most often external for frogs but internal for salamanders and caecilian.
• Most salamanders use their sense of smell to find a mate, whereas frogs use mating calls.
• In salamanders, internal fertilization occurs in the cloaca during the few minutes preceding oviposition. Thus, while fertilization is internal, embryo development is entirely external.
• In frogs, external fertilization occurs where the female releases the eggs into water. Then, the male releases his sperm to fertilize them.