If the virgin Mary was chosen to be the mother of Jesus why is there so little about her in the Bible?

As the Scriptures themselves tell us, not everything is written down, and the New Testament was not written until many years after Our Blessed Lord's assumption into heaven, nor formed, as we have it today, until centuries later. The Bible, specifically the New Testament, was written as a preaching tool, NOT as a compedium of Christian doctrine. No dogma may contradict Scripture, but it is not all contained in Scripture, that is a protestant heresy.

Answer:

Because the central or key character in the entire Bible is Jesus Christ - from the Creation in Genesis to the return establishing the Kingdom of God on Earth in Revelation. He is the God Family member who is putting God's Plan to re-create Himself in a family of sons and daughters in action. No other individual from Adam to the last person born of men is as vital as Jesus Christ. Indeed, the Scripture tells us numerous times that the only path of Salvation is through Him - our only intercessor. As Scripture was completed before the end of the 1st Century AD via John's last Book of Revelation, we read in Jude 3 of our common salvation via Jesus Christ alone as '...that ye should earnestly contend for the faith which was once delivered unto the saints.'