Peter Adam writes:
(1) Preaching through the books of the Bible, verse by verse, chapter by chapter, respects and reflects God’s authorship. God did not gives us a book of quotable quotes, nor a dictionary of useful texts, nor an anthology of inspiring ideas. When God caused the Scriptures to be written the medium that he used was that of books of the Bible. If that was good enough for the author it should be good enough for the preacher.
(2)Expository Preaching reflects God’s respect for human authors. One of the most beautiful features of the Bible is the way in which God causes his truth to be written and yet does not over-ride the individual writer, but respects their place in history, their vocabulary, their spoken and literary style. If God is so careful to respect the human authors of the Scriptures we should endeavour to do the same by reading, studying, preaching and teaching their books in the order in the way in they wrote them.
Read the other thirteen reasons and more in Peter Adam - Arguing for Expository Preaching