In every gospel dispensation this has been the case. Adam, Enoch, Noah, Abraham, Moses-all received from the Lord not only their prophetic commission but also continuing revelation suited to their time and people.
The Book of Mormon tells the same story. Revelations given to former prophets were honored as true and as significant in teaching and inculcating saving principles; but they were not of themselves sufficient to guide and sustain the current body of believers and bring them to eternal life.