Question

Why is the middle age called the barbariwn age of human development​

Answer 1

It's because from this age human have become civillzed and developed various infrastructure

Answer 2

Today, the adjective “barbaric” is most commonly used to describe an act that is either brutal or cruel to the point of savagery or primitive and uncivilized (or all of the above) while a “barbarian” is a person who commits such acts or displays such characteristics.