How about because his audiences were made up of peasants and illiterate townsmen? How many man-in-the-street accounts do we have of *any* ancient person or event?

The simple fact is Jesus, during his lifetime, was one of any number of religious teachers with a following in Judea and so of no particular note to his literate, and mostly Romanized, contemporaries. It should however be said that later Roman writers dealing with the early Christian movement did not doubt his historical existence however contemptuous they might be of his followers and his teachings.