It was because only human and monkeys can kill Ravana.
Hanuman is a Monkey God.
hanuman
The monkey god is typically called hanuman or "Lord Hanuman." He is one of the most popular gods in Hindu mythology.
Hanuman
Hanuman
Hanuman is a very powerful and strong god. He finds an important place in the Ramayana. He was a devotee of Rama, a form (avatar) of Vishnu, a god of Hindus. Hanuman is a figure of strength, perseverance, and devotion. Hanuman was the son of Vayu, the god of the wind, and Anjana, a celestial nymph. You're probably wondering how the son of the wind god and a nymph turned out to be a monkey. The answer is that Anjana once angered a sage who cursed her to be born as a monkey. ... Hanuman fell to earth, wounded.
hanuman ji ,the devotee of Lord Ramachandra , lived in the monkey city of Kiskindha
a human body in the form of a monkey
Hanuman
Hanuman
Darvin theory of revoluation is true. Human arrive from monkey.
Hanuman