How could they NOT have known?

He grew up before him like a tender shoot,
    and like a root out of dry ground.
He had no beauty or majesty to attract us to him,
    nothing in his appearance that we should desire him.
³ He was despised and rejected by mankind,
    a man of suffering, and familiar with pain.
Like one from whom people hide their faces
    he was despised, and we held him in low esteem.

⁴ Surely he took up our pain
    and bore our suffering,
yet we considered him punished by God,
    stricken by him, and afflicted.
⁵ But he was pierced for our transgressions,
    he was crushed for our iniquities;
the punishment that brought us peace was on him,
    and by his wounds we are healed.
⁶ We all, like sheep, have gone astray,
    each of us has turned to our own way;
and the Lord has laid on him
    the iniquity of us all.

⁷ He was oppressed and afflicted,
    yet he did not open his mouth;
he was led like a lamb to the slaughter,
    and as a sheep before its shearers is silent,
    so he did not open his mouth.
⁸ By oppression^[a] and judgment he was taken away.  Isaiah 53

It has been said that Hebrew people were adept at memorizing the Hebrew bible; that

it was a certainty that young men, like Jesus, had memorized large portions of the bible

in their educational training. 

With that in mind, it seems amazingly ridiculous that Jesus was not recognized as the

Messiah when there were numerous Old Testament verses that clearly prophesied

both his coming and His death.

The disciples seemed, at times, confused and unclear as to who this Jesus was. Simon
Peter responded to the question by stating that "Thou art the Christ, the son of the
living God." Matt 16: 16 Yet they continued to seem dazed at events and His words.
It's true that the Hebrew people were, and still are, waiting for a messiah who would
come in power to save the nation Israel and overthrow the Roman government. Obviously,
Jesus arrived unspectacular and was characterized more for compassion and
servitude. He was carrying a powerful message in a humble way.

So what are we to make of this inability of those present to recognize who Jesus was?
He minced no words in directly claiming to be the Son of God; yet, they ended up calling
for His death.  Then Jesus said to them, “Most assuredly, I say to you, before Abraham was, I AM.” John 8: 58

I believe that the answer must surely be that God was working for His good plan by
making it impossible for the Jewish leadership to recognize what they most clearly
sought, the Messiah. There were just too many signs that this was the Son of God; yet,
He was destined to live out His purpose by being crucified and raised in victory over
Satan and death. It was God's plan, and like everything else that God does, it was good!

Praise our God!