How come...

If, in every dispensation, Heavenly Father or Jesus commands the people to write a record of what is happening, why is our record of the time when Jesus was on the earth not written until 60-70 A.D.? This is such an important time, i would think He would not want to trust the memories of the apostles for that long a time. When He came to america, he says, why didn't you write the account of samuel the lamanite? Surely His actual teachings and activities and church setting-up would be important to get right. right now. someone write this down.