The Gospel frees us to live in Christ, not in our own strength. Therefore preachers should be able to preach by the grace of God. This could easily be left as an amorphous reliance on God in sermon preparation, but I think that it can be made concrete in expositional preaching.
Consider: a preacher may rely on himself as he crafts his sermon. He may work out his own structure and his own topic for what he says, and find texts from Scripture on his own. He may choose his own tone, his own words, his own conclusion. Or a preacher may rely on God, by taking a text and preaching its structure, which is the structure of the Word of God and so God's structure. He may preach the topic of that text, the tone of that text, the keywords and phrases of that text, and the conclusion of that text. He may look and see what God has already given him to preach. "Here is what God says" the preacher may then say, "I will tell my congregation about it." He then locates the text in the way God's word is organized around the Gospel of Jesus Christ, and then preaches what this text says, as it says it, to the end for which it says it.
This is why I am amazed by how little expositional preaching I find, as I look for a church. expositional preaching is, to me, the obviously best way to preach from Scripture, as well as the easiest. God has already done the work. Why try to reinvent the wheel? Why not preach off of what God--who is way better at this stuff than any of us--has already done? Why make things harder for yourself than you have to, especially when it is likely to have worse results?
There are various possibilities as to why preachers don't preach this way. Perhaps some don't preach this way out of ignorance: it may just never have occurred to them that they can, and no one has told them. Some may have worse reasons. Expositional preaching requires that the preacher submit himself to the word of God, and that requires humility. We are proud and want to do things for ourselves, but you know what? Christ died so that it is no longer about what we do, but what Christ has already done. You don't need to run around making your very own pretty sermon. God has already given his message, you don't need to make one up for him, or try to find some secret message. Maybe there is fear. Of what? That it will be boring? Because it is a new way of preaching? God's word is not boring--the Gospel of Christ is the most exciting story in the world, the one all other good stories image, the one we are all hungry for. It is new, though. But of all kinds of preaching to try for the first time, expositional preaching is the best to mess up. Pick a book; work through it. Even if it is the crappiest sermon you have ever preached, God's word lies behind it. God still speaks, and he speaks even in our weakness, even in if the weakness is in the area of preaching.