Why do many scholars think Mark was a source for Matthew and Luke?

In the field of literary criticism, the theory that the Gospel According to Mark served as a source for the Gospels of Matthew and Luke is known as the priority of Mark. This conclusion is central to the generally accepted two-source hypothesis.

Scholars base this conclusion on several key observations:

• Structural Patterning: Matthew and Luke follow the basic chronological outline and narrative structure of Mark. When Matthew and Luke deviate from Mark’s order, at least one of them typically aligns with Mark’s sequence.12
• Literary Dependency: There is extensive word-for-word agreement among the three Synoptic Gospels. Crucially, Matthew and Luke rarely agree with each other against Mark; rather, they agree with Mark individually.12
• Editorial Refinement: Mark is written in a simpler, more direct, and sometimes "cruder" Greek style. Scholars observe that Matthew and Luke appear to have "improved" or polished Mark’s language while incorporating his material.23
• Content Overlap: More than 90 percent of Mark’s content appears in Matthew, and over 50 percent appears in Luke.42