Baha’u’llah found the “generality of mankind…still immature” and not having “acquired sufficient capacity” to receive a fuller measure of His Revelation

The generality of mankind is still immature. Had it acquired sufficient capacity We would have bestowed upon it so great a measure of Our knowledge that all who dwell on earth and in heaven would have found themselves, by virtue of the grace streaming from Our pen, completely independent of all knowledge save the knowledge of God, and would have been securely established upon the throne of abiding tranquillity. 

- Baha'u'llah  (From a Tablet, quoted by Shoghi Effendi in ‘The Dispensation of Baha’u’llah; included in ‘The World Order of Baha’u’llah’)