Because it was an in your face revelation. It was proof that God still talks to man. It was the catalyst to re- start the Church of Jesus Christ.
As an EV convert it gave me a clear picture of where the United States place was in the end time prophecies
This was something that had bothered me for two years while studying the Old Testament I could not figure out where we fit.
I'd like to add that I think everyone that was born in the church doesn't understand how it feels to be boxed in with only the Old Testament and the New Testament with no other scriptures. I truly do treasure the Pearl of Great Price, The Book of Mormon and the Doctrine and Covenants but the Book of Mormon was the catalyst to bring these others about.