Why did the law of Moses, important for many generations, eventually become “dead”?

not in 2 Nephi 12:9 clarify the corresponding verse in Isaiah 2:9? The Book of Mormon clarifies the corresponding verse in Isaiah 2:9 with the addition of the word not in two places. This verse shows “that the ordinary man and the great man do not humble themselves before God” (Parry et al., Understanding Isaiah, 29n15).