Good question. Basically the manuscripts would not have survived the ravages of time. There are two reasons why.
First, we have to remember that the conditions then are not like the conditions now. They didn’t make paper then as we do now. The raw materials and process at that time did not create good paper and it disintegrated very easily. Since they were not as scientifically advanced as we are today, they also did not know how to care for manuscripts, which added to the quicker disintegration of the documents. So while we might not have the original manuscripts, we might have copies of copies of them.
Second, we have to remember that many religious wars along the years – many of them sought out to destroy religious manuscripts and artifacts. Clerics and scribes would have probably hidden these in secret caves or tunnels, many of which have been forgotten or lost.