We try to convert other Christians to complete the gospel of Jesus Christ.The book of Mormon is not a different or stand alone \"Mormon Bible\" it is the \"Book of Mormon Another Testament of Jesus Christ\". 
There is more revelation given to a modern day prophet that we want to share the joy of that new revelation just like christians want to share the New Testament with non beleivers. The World is ever changing, don\'t you think God wants to give us new information to help lead us through this life? I do. Amen.