One theory that I like (and yes I know that I am being heretical and would be drummed out of any ELCA seminary for putting forth such an impossible theory) is that the sayings that are common to Matthew and Luke were witnessed by Matthew so he had them first hand and that Luke in his research spoke with eye witnesses who related them to Luke. Why are they not in Mark? Either Mark didn't talk with the same people, or that Mark thought that they were not important for the narrative and points that he was making and so left them out. Less writing for him to do, less stuff for his readers to plow through. Alternatively, in making his condensed Gospel and since Matthew and Luke had already covered the material, he didn't think it important to include it.
The Gospels were not scholarly biographies as we know the genre today. Today a scholar will want to write a comprehensive overview of the entire life of his subject and include as much of the material that he researched as possible. Even so, there will be sifting of material and selecting of what is important to include and what can be omitted since even scholarly biographies can be only so long. These are Gospels, designed not to give complete biographies but to assemble the material important for creating faith and guiding the faithful, according to the points that the Gospel writer intends.
Perhaps Q was not so much a document or an oral collection so much as the body of witnesses that the Gospel writers consulted. Let's not forget that the scholarly consensus places the writing of the Gospels in the later half of the first century, at most some 70 years after the death of Christ. If some of the eye-witnesses to Jesus' ministry, death and resurrection were young adults at the time, that places the writing of the Gospels within a long lifetime of the eye witnesses, children of eye witnesses at most. We are not talking about a couple of hundred years of oral transmission before the written Gospels were produced. And in an era when literacy was less common, oral tradition was conservative, prizing accuracy more than innovation.