Why do Christians not engage in the Great Commission? Unbelief. If Christians believed that men were sinners and believed in the power of the Gospel to save sinners they would take action to alleviate mankind from sin by the preaching of the Gospel.
The supreme evidence of sin is unbelief. Only by prolonged and deep sanctification which produces mature Christians does a Christian believe that man is a sinner and that the Gospel is the singular power to save.
The Apostle Paul represents to Gentiles this trajectory: he was a legalist and persecuted the church; then Jesus saves him on the road to Damascus; Paul, then Saul, preaches in the Damascus synagogue that Jesus is the Christ; but then he disappears to be taught the Gospel by direct revelation of Jesus Christ where he is brought to the place of belief in the Gospel.
Paul believes the Gospel is the power to save souls from sin and he lives his life preaching the Gospel because it is the power to save men from sin.
Do you believe men need to be saved? Do you believe in the power of the Gospel to save them? If yes then what are you doing about it? How are you engaged in the work of the Gospel?