I have heard this question a lot of times! Why would we need more if God has already given us this much? Well, first off, this life is ALL about progression. God wants us to continue to learn, grow, develop, and become more than we are today. So, He teaches little by little.
Isaiah 28:10
"For precept must be upon precept, line upon line, Here a little, and there a little."
So God wanted us to learn step by step. He feels that we have learned what we have needed to learn from the Bible and is ready to give more.
Another struggle is that people have changed those important things in the Bible and now the Bible is not complete. The Bible has even predicted this.
Isaiah 24:5
"The earth also is defiled under the inhabitants thereof; because they have transgressed the laws, changed the ordinance, broken the everlasting covenant."
Well, God is a God of order and great knowledge and understanding. So he knew that this would happen and so he had people in the Ancient America's keeping record so that He could bring it forth when it was time to bring the fulness of the gospel back.
Christ said,
"Wherefore, I the Lord, knowing the calamity which should come upon the inhabitants of the earth, called upon my servant Joseph Smith, Jun., and spake unto him from heaven, and gave him commandments."
"And also those to whom these commandments were given, might have power to lay the foundation of this church, and to bring it forth out of obscurity and out of darkness, the only true and living church upon the face of the whole earth, with which I, the Lord, am well pleased..." (D&C 1:17, 30)
God has prepared the way for the gospel to come back to the earth and to have those sacred things restored to the earth. The time for this to happen is now. The time is called the "fulness of time."
Ephesians 1:10
"That in the dispensation of the
fulness of times
he might gather together
in one all things in Christ,
both which are in heaven,
and which are on earth; even in him."
We live in the fulness of time, and God has brought the gospel of Jesus Christ through the man who was called as a prophet of God, Joseph Smith. The Book of Mormon clarifies and refines the doctrine of God and what we have to do in order to return to God.
I know that this is true. The Book of Mormon does not take away anything from the Bible, it only clarifies and helps us understand who Jesus Christ is, what we need to do, who we need to become, and how we gain peace and happiness in this life.