Why does a tadpole develop into a frog?

Essentially, the larval stage in a species' development cycle renders a specimen as a different type of lifeform than the adult stage does. Tadpoles resemble small fish such as minnows much more than adult frogs so they are labelled as larvae. Mammals look pretty much the same throughout life, so they are not said to have a larval stage. Larvae feed rapidly to fuel the metamorphosis into adulthood. See the related Wikipedia link for a much more detailed explanation.