One reason is that it would have REALLY confused the locals! Imagine Jesus telling a good monotheistic Jew of His day that He was God. If the hearer...
ACCEPTED that fact, then he would have lost "God the Father"! Jesus had an INCREDIBLY fine line to walk here--too much emphasis on His deity, then...
monotheism (or God the Father) would have disappeared; not enough communication on His deity, then the significance of His...
humiliation/sacrifice/exaltation would have disappeared. Does this seem like a reasonably possible explanation to you?
If the person asks where's the proof for God/Jesus/Christianity, go here.