Eve was deceived into sin, and sinned. Adam was not deceived, and yet sinned. The Bible says through one man, Adam, sin entered into the world. Eve was tempted by a serpent, satan, to eat of the tree of the Knowledge of Good and Evil. It's that simple...it was a real tree. We will all one day eat of the tree of life. Women and Men are both sinners, but it is the seed of Adam that will be passed on to future children. I am a sinner, because I am a daughter of Eve by the seed of Adam. Women bear the children of men, they were made for man. As the weaker vessel, we are easily deceived, and that is why satan went to her instead of recognizing Adam as her head.
So, since Adam's seed are born into sin, and Jesus was not born of the seed of man, but of God, Jesus was not born into sin, and He did not sin. All the rest of us are born into sin, and sin. Jesus was to be the Lamb of God who takes away the sin of the world...He knew no sin. He was also to be our King, and so He was Royalty through Joseph (He had Joseph's family name) and Mary who were of the house of David, and through God...King of kings.
Mary was a virgin for the sake of purity...would the Holy Spirit come upon a woman who already knew a man? No, not in a womb that has already born the seed of Adam. It was also a matter of witness to us, a birth from above, a miracle. It was also a picture of our own rebirth in the Spirit. He had to be born, to be in the flesh to die and rise again. He had to be a Priest of no lineage He could not have a physical earthly father and qualify for an eternal priesthood. Jesus also had to be a Jew. So, He was born to a Jewish family, through a Jewish virgin ,so that He would be of the chosen race and be able to keep the Law, and fulfill the Law. They were entrusted with the written word of God, and the Word of God. They were given manna from heaven, and He is Manna from Heaven. Jesus was to be a Prophet, and so He was born of God to be the Revelation of God to us. This was all important for the covenant to be changed. It was God's plan, and it was important for our redemption. Again, Jesus was the Lamb of God. There is no remission without the shedding of blood, and as Cornelius said the life is in the blood...His blood was pure and holy, and He alone was worthy to drink from the bitter cup, and through His shed blood we have life.
There are probably more reasons, and they would all fall under the umbrella of prophecy and a fulfillment of God's plan of love.