The first disciples of Christ were simple fishermen, not learned men. Among those who first heard the Sermon on the Mount, there were hardly many educated people. But for some reason it is precisely these unwise and weak ones that Jesus Himself chooses in order to entrust to them His chief mystery.
The Savior's grateful exclamation was prompted by the fact that seventy of His followers came to Him and joyfully reported that "even the demons are subject to us." The childlike simplicity of their faith, with which they receive what the Lord tells them, and the directness with which they express their joy at the greatness of God's works, are simply incredible. If they had been full of clever reasoning and doubts, if they had reasoned "like adults," then the power to work miracles in Jesus' name would hardly have been revealed to them.