It is too difficult, overwhelming. Despair leads to hopelessness and hopelessness to tunnel vision. These missionaries can only see what is right before them. They fail to look above and fathom God's presence and mission to the world. Neither do they see below to comprehend both Satan's power and God's all-surpassing power over all principalities and powers.
Some missionaries minister primarily out of ego.
They believe that they will accomplish great things because they have made great plans, because they work creatively and ingeniously. They are quick to tell all who will listen that their work is successful because they have made it so. They lack the humility to realize that, in reality, they can do little by their own power and ability. Paradoxically, the responses of despair and ego have much in common. Both are human responses to God's mission, and both originate in human pride.