The simple succinct answer to this is that he couldn’t take the chance of tipping his hand to Satan. Had Satan figured out what the crucifixion was going to accomplish, he would have aborted it. Then why, in the very same chapter of John, did Jesus tell Jews in the temple of Jerusalem that he WAS the son of God? He said it to them, just not to Pilates.
John 10:17-18 For this reason the Father loves me, because I lay down my life that I may take it up again. No one takes it from me, but I lay it down of my own accord. I have authority to lay it down and authority to take it up again. This command I received from my Father."
Then why, in the very same chapter of John, did Jesus tell Jews in the temple of Jerusalem that he WAS the son of God? He said it to them, just not to Pilates.
Matthew 26:55
At that hour Jesus said to the crowds, “Have you come out as against a robber, with swords and clubs to capture me? Day after day I sat in the temple teaching, and you did not seize me. 56 But all this has taken place that the Scriptures of the prophets might be fulfilled.” Then all the disciples left him and fled.
Mark 14:48
And Jesus said to them, “Have you come out as against a robber, with swords and clubs to capture me? Day after day I was with you in the temple teaching, and you did not seize me. But let the Scriptures be fulfilled.” And they all left him and fled.
Luke 22:52
Then Jesus said to the chief priests and officers of the temple and elders, who had come out against him, “Have you come out as against a robber, with swords and clubs? When I was with you day after day in the temple, you did not lay hands on me. But this is your hour, and the power of darkness.”
John 18:19
The high priest then questioned Jesus about his disciples and his teaching. Jesus answered him, “I have spoken openly to the world. I have always taught in synagogues and in the temple, where all Jews come together. I have said nothing in secret. Why do you ask me? Ask those who have heard me what I said to them; they know what I said.”
The Sanhedrin requests, "If you are the Christ . . . tell us."
In sum, Jesus makes himself and his authority the issue. The leaders are astute enough to see the claim. So they ask, "Are you then the Son of God?" They sense the depth of what Jesus is claiming--that Jesus uniquely shares God's rule and power.
Jesus' answer is both a positive reply and a type of circumlocution; in effect, he says, "I will not deny it, but I would mean it a little differently from the way you mean it."
So Jesus says, "You say that I am" (Blass and Debrunner 1961:par. 441.3; Catchpole 1970:226; Stein 1992:571). The NIV gives the force of this indirect reply: "You are right in saying I am."
So the judgment is rendered against Jesus. "Why do we need any more testimony? We have heard it from his own lips."
http://www.biblegateway.com/resources/commentaries/IVP-NT/Luke/Trial-Death-Jesus