With the emergence of invention and discovery and the development of institutions, the human mind is bound to become more and more open and the human brain is getting bigger and bigger, especially the part of the brain. In ignorance stage, it is extremely difficult for people to come up with the simplest inventions from an environment with nothing. 
the slow development of human mind at that time was inevitable. Of course, it is also very difficult to organize such benighted and unruly people to form a society. There is no doubt that the earliest invention projects, the earliest social organizations, are difficult to produce, so the transition from ignorance stage to barbaric stage takes the longest time.
In the stage of ignorance, there are no inventions, discoveries and institutions to speak of. These three are the necessary conditions for the beginning of a barbaric society, so there is a long interval between them.