Because one wicked people (false religious leaders and populist politicians) understood that Jesus was the Christ his death was a certainty.
Every time that Jesus did a miracle that proved he was the Christ, he would tell the recipient to please keep it quiet. This was for his own safety!
Every piece of public credibility proving that Jesus was the Son of God was another nail in his coffin, another signature on his death warrant, another step towards his termination.
The more credible he became to the wicked as the Son of God, the more inevitable was his demise.
The final straw for the Sanhedrin was his public resurrection of Lazarus
47 Consequently the chief priests and the Pharisees
gathered the Sanhedrin together and began to say: What are we to do, because
this man performs many signs?
48 If we let him alone this way, they will all put faith in him, and the Romans will come and take away both our place and our nation.
49 But a certain one of them, Caiaphas, who was high priest that year, said to them: You do not know anything at all,
50 and you do not reason out that it is to your benefit for one man to die in behalf of the people and not for the whole nation to be destroyed.
51 This, though, he did not say of his own originality; but because he was high priest that year, he prophesied that Jesus was destined to die for the nation,
52 and not for the nation only, but in order that the children of God who are scattered about he might also gather together in one.
53 Therefore from that day on they took counsel to kill him. (John 11 NWT)
But the focus of his show trial before the Sanhedrin was whether or not he was the Son of God...
63 But Jesus kept silent. So the high priest said to
him: By the living God I put you under oath to tell us whether you are the
Christ the Son of God!
64 Jesus said to him: You yourself [have] said [it]. Yet I say to you men, From henceforth you will see the Son of man sitting at the right hand of power and coming on the clouds of heaven.
65 Then the high priest ripped his outer garments, saying: He has blasphemed! What further need do we have of witnesses? See! Now you have heard the blasphemy.
66 What is your opinion? They returned answer: He is liable to death. (Matthew 26 NWT)
Jesus was declaring that they had already said that he was the Son of God, the Christ. Their concern was that he was indeed God's son and that he now had sufficient public credibility to undermine their position as religious leaders and take away their livelihood and status and wealth. So being wicked self interested pigs they killed the Christ and kept their jobs for a few years.
This is why Jesus told his disciples not to reveal to anybody that he was the Christ (God would reveal that only to those who were not wicked)..
20 Then he sternly charged the disciples not to say to anybody that he was the Christ. (Matthew 16 NWT)
And more to the point that is why the demons kept declaring it. The demons wanted him to be publicly known to be the Christ so that he would be killed and his ministry cut short!
23 Also, at that immediate time there was in their
synagogue a man under the power of an unclean spirit, and he shouted,
24 saying: What have we to do with you, Jesus you Nazarene? Did you come to destroy us? I know exactly who you are, the Holy One of God.
25 But Jesus rebuked it, saying: Be silent, and come on out of him! (Mark 1 NWT)
11 Even the unclean spirits, whenever they would behold
him, would prostrate themselves before him and cry out, saying: You are the Son
12 But many times he sternly charged them not to make him known. (Mark 3 NWT)
41 Demons also would come out of many, crying out and saying: You are the Son of God. But, rebuking them, he would not permit them to speak, because they knew him to be the Christ. (Luke 4 NWT)
It has been the same throughout Christian history. If the true church was
known it would have been persecuted out of existence. Indeed that is what
happened to TCC2 which was destroyed by the Roman Catholic church. So that is
why the bible is coded. The code of the bible is the cover which enables a true
church to operate without being perceived as much of a threat by Caesar.
Although actually the true church is no threat to Caesar. It is only a threat to
false churches - who lobby Caesar against the true church.