Not “why?” but “why not?”
From the book The creation of a student movement to evangelize the world by Timothy C Wallstrom
“Suppose then that the individual Christian does have an obligation to tell others of Christ,” a student might respond, “why must he go overseas to do so? Is there not ample need at home?” The volunteers answered this objection in two ways. First, because the missionary effort is so patently an enterprise of faith, its maintenance will indeed exercise and strengthen the spiritual vitality of the home church, much as the use of the bodily extremities will exercise and strengthen the heart which supplies them with blood. Missions do not sap the vigor of the home base; they fortify it.
The individual obligation to minister overseas was in fact a simply corollary of this greater need; no special call was considered necessary. Robert Wilder, one of the Movement’s originators, found “nothing in the Bible to indicate that a man needs more of a call to take him to Africa than to Dakota.” The whole of a Christian’s existence was held to be consecrated to the service of his Master, and the sole remaining question involved the means through which that service could be most effectively rendered. In the presence of a specific command and an urgent need to minister abroad, volunteers believed the duty rested with the individual to show that his life might be used more effectively in other ways. It was not God’s responsibility to push him in the direction to which He had already pointed. Volunteers believed the Christian attitude to the missionary call should not be “why?” but “why not?”