“[My word] shall not return to me void, but shall do my will, achieving the end for which I sent it.” Christ, the Word of God, was sent to do the Father's will and achieve a particular end: our salvation. Along the way, He cured some illnesses but not all. He drove out some demons and raised some people from the dead: but not all. He did not refuse those who came to Him for that, but He did not leave behind Him a world freed from every sickness and trouble. That was not then His mission. Like Christ, we too have a particular mission, and we live it out amidst the need and trouble that meet us almost at every turn. We cannot do everything, and trying is more likely to end up with us doing nothing.
Another purpose of Lent is to follow Christ into the silence of the desert – a place apart, where the noise of the world does not come. It's a place to ready ourselves, to more clearly hear what the Lord asks. “I will lead her into the desert and speak to her heart, and there she shall answer.”