Given that around 90% of Mark is represented by Matthew and that Luke incorporates around 60%, why did early Christianity keep Mark around at all? The venerable Larry Hurtado discusses this question and suggests an answer.
I might also add that we have numerous examples from the ANE where texts were cannibalized. That is, they were incorporated, authorship was "reassigned" and the primary text was destroyed. Notice, however, that we find at least two remarkable exceptions to this in Jewish scripture. We have overlapping traditions set side by side in the Pentateuch and a similar relationship demonstrated between Samuel/Chronicles. The presence of these texts in the Qumran library suggests that these concomitant shelf-lives are not merely explained by geographical preference. I might also add to this list the case of Jude/2 Peter. Could it be that there was an element of "sacred preservation" in Jewish culture that Christianity inherited?