Thomas Schreiner explains:
“Why did Jesus use the title ‘Son of Man’ so often and in comparison, relatively speaking, the title ‘Messiah’ so little? We have seen that the title ‘Messiah’ was apt to be understood in militaristic and political terms, contrary to Jesus’ mission. The term ‘Son of Man,’ on the other hand, was ambiguous. Even though the term was used in the OT, there was some uncertainty about its meaning. We have seen that typically in the OT ‘son of man’ simply means ‘human being.’ Even in Dan. 7:13-14 ‘son of man’ represents a humane and civilizing kingdom in contrast to the bestial kingdoms that maltreat human beings.
Jesus preferred ‘Son of Man’ because he could pour his own content into it. We see from John 12:34 that Jesus’ contemporaries were perplexed and puzzled by his use of the term. By using this term, he did not automatically arouse suspicion and antagonism, and he could slowly teach his followers the significance and meaning of ‘Son of Man.’ Their understanding of ‘Son of Man’ could be reoriented as Jesus’ teaching and ministry progressed.”