In essence, Jesus answered this same question by saying that He did not do so because He had to die.  Our question then boils down to this:  Why did Jesus have to die?  I can think of six reasons, and there are likely more, but we will focus only our attention on these.
    First, because God would not ask someone else to do something that He Himself was unwilling to do.  In Genesis 22, we find an occasion where God has told Abraham to offer his own son, God likewise would offer His.
    Second, because prophecy demanded that he die.  From Matthew 3 through the Gospels themselves and even into Revelation we see evidences of prophecy demanding that the Christ die on the cross.  These prophesies had to be fulfilled or fail.  They were fulfilled.
    Third, remission of our sins required the shedding of blood.  Hebrews 9:22 plainly states, "And almost all things are by the law purged with blood; and without shedding of blood is no remission."  Someone had to die, and that someone could only be the perfect Christ.
    Fourth, God's love made it necessary.  When sin endangered the world, God by means of His Son restored hope..  John 3:16 reads, "For God so loved the world, that he gave his only begotten Son, that whosoever believeth in him should not perish, but have everlasting life."
    Fifth, God's justice dictated it.  This is similar to our second reason, but extends beyond on this point:  Not only had the Messiah been prophesied, He had been promised and His death likewise so.  When God through inspiration penned the prophesies He was penning the inevitable.  Genesis 3:15 was a promise, Isaiah 53 was a promise.  That is why without doubt Job could say in Job 19:25, "For I know that my redeemer liveth, and that he shall stand at the latter day upon the earth:"
    Sixth, because the grave could not hold Him.  What Christ would suffer would be short lived.  Luke 24:46 shows that "it behoved Christ to suffer," but likewise "to rise from the dead the third day."