1) His mistakes outweighed his successes!
2) So that duff sinners like you and me could have an example to encourage us. Peter was a headstrong, at times mildly stupid, sinner. Peter is a lot like me. Yet Jesus accepted Peter and the two of them were great friends. Jesus hung out with Peter despite his sin, encouraging Peter to trust Him and to love Him as "the only name given under heaven by which men can be saved" (Acts 4: 12). Furthermore Jesus used Peter loads, Acts 2-10 are full of examples of this, as is the fact that two of Peter's letters are in the New Testament.
Calling Peter a rock, when Peter was more like jello, was deliberate. Peter must have known he couldn't live up to his new identity on his own - he needed help.
It's the same for us - we can't live the Christian life in our own strength. It calls us to be people we cannot be in our strength. What encouragement there is to know that God uses Peter, me, you and every other justified and adopted sinner to build His church. The church of God will always be built upon jello-like "rocks" whom God has quarried out from the dominion of darkness and placed in His kingdom of His beloved Son. This is God's plan, because through our wobbly weaknesses the steadfast beauty of Christ is most clearly seen, (2 Cor 4: 7).
And so just like Peter we need help from the true Rock. We need His grace to live.
"I love you, O Lord, my strength.
The Lord is my Rock and my Fortress and my Deliverer,
my God, my Rock, in Whom I take refuge,
my Shield, and the Horn of my Salvation, my Stronghold."
Ps 18: 1- 2