Off the top of my head, I think it is because Mark is basic only 16 chapters. I know these are 4 independent Gospels because they are by 4 individuals, but the first one, Mark, is like the first draft so it doesn't include all the details.

And John's objective is not just repeat what was already said in Matthew and Luke, but to set down many words not found in the other Gospels. The 4 Gospels are very efficient in their allocation of information by pen and paper. Information included in one and not another shows the independent accounts. The Gospel of John is the most profound of all the Gospels as well as the last written. It was written after most of the rest of the New Testament was composed.

Other Gospels and many Epistles had already appeared, but at the very last, John came forth to present his Gospel. In it there is finally shown to us what God's estimate of Christ is and in it we are told how we ought to know Christ as God knows Him deeply. We see that Christ is all these things-lamb, bread of life, the way, the truth, the life, power, and more. That He is not simply going to give us these things. He says He is these things. So John's Gospel is even more profound with different emphasis. It really rounds out the 4 Gospels I feel and feels intuitively appropriate the virgin birth is not included. Besides, you don't want everything repeated verbatim.