The reason is that God, for his own glory and purposes, chooses to use means to accomplish his will. ("Means" are not ill-tempered, cranky people!) Means, rather, are the ordained ways by which God fulfills his preordained purposes.
For example, we know that Christ has purposed to return some day. He will return no matter what. All that he has ordained to come to pass between now and then (missions, the preaching of the gospel, the perseverance of the church, the "fullness of the Gentiles" being brought in, etc.) will also come to pass, no matter what. Yet he has told us to pray for his return ("pray like this... 'Let your kingdom come and your will be done on earth as it is in Heaven.'") He told us to pray for all that comes in between. We are to beseech the Father to send laborers into the harvest (missionaries, church planters, teachers and preachers) and to pray for the church, for her success in proclaiming and living out the gospel in the world, and to pray and work for our own sanctification, even though "it is God who is at work in you, both to will and to work for his good pleasure" (Philippians 2:12-13).
In his wonderful little book The Bruised Reed, Richard Sibbes offers this helpful illustration, showing that the use of means is a principle taken for granted even in civic matters: "For who, if he knew before that it would be a fruitful year, would therefore hang up his plough and neglect tillage?" In other words, if a framer could somehow see ahead and know that harvest time will bring a rich crop, he would still get his plow out in the spring to till up the ground and plant the seed. His plow and his effort and the seed are the ordained means to the certain, coming bounty.
So God, in the unsearchable riches of his wisdom and ways, has ordained that all his sovereign purposes will surely come to pass, and that they will not come to pass apart from the prayers and labors of the saints. Again in the words of Richard Sibbes, "The means that God has set apart for the effecting of any thing are included in the purpose that he has to bring that thing to pass." We can be sure that in whatever he has decreed, he will also provide the means--whether prayer, or the preaching of the gospel, or any other ordained means--to accomplish those purposes.