The reason is because Jesus is not God. Jesus never claimed to have the same knowledge as God the Father:
"Concerning that day and hour nobody knows, neither the angels of the heavens nor the Son, but only the Father." (Mt. 24:36)
"Jesus went on to say to them: “Most truly I say to YOU, The Son cannot do a single thing of his own initiative, but only what he beholds the Father doing." (John 5:19)
"What I teach is not mine, but belongs to him that sent me." (John 7:16)
"Jesus said: “I do nothing of my own initiative; but just as the Father taught me I speak these things." (John 8:28)
"For from God I came forth and am here. Neither have I come of my own initiative at all, but that One sent me forth." (John 8:42)
"I cannot do a single thing of my own initiative; just as I hear, I judge; and the judgment that I render is righteous, because I seek, not my own will, but the will of him that sent me." (John 5:30)
"The Father is greater than I am." (John 14:28)
Some may argue that Jesus only said these things because he was only a man when he said them. But even after Jesus went to heaven he was described as having a subordinate role to God:
"But I want YOU to know that the head of every man is the Christ; in turn the head of a woman is the man; in turn the HEAD OF CHRIST IS GOD." (1 Cor. 11:3)
For more, see:
Is Jesus God?
Why Don't Jehovah's Witnesses Believe in the Trinity? (VIDEO)
Examining Trinity 'Proof Texts'
Search For Bible Truths - ARCHIVE
Search For Bible Truths - Search Guide