The Gospels are not meant to be simply factual reports, but bring out the theological significance of real events for their intended contemporary audience. So Matthew places Jesus’ sermon on a mountain in order to bring out a connection for his original Jewish audience.  Jesus speaking on a mountain about the law would remind them how God gave Moses the law on Mount Sinai (Exodus 19-24): Luke, written primarily for Hellenistic Christians, would not pick up on that connection so readily.