Lehi presents the best evidence possible in that far off land - a vision. Lehi declares now that Jerusalem is destroyed, not that it would be destroyed as he had in the past. This is a shift in the message. While Jerusalem stood, Laman and Lemuel could assume that the prophecy was in the distant future, and therefore was no reason for their departure. Lehi quells that issue. Jerusalem is destroyed. The original land of inheritance is no more, and now they must work with the land of promise.
Historical Background: The destruction of Jerusalem referred to in 2 Nephi 1:4 is recorded in the Bible in 2 Kings 25. Lehi and his group had been warned by the Lord to flee from the land of Jerusalem so that they would escape this destruction. Most biblical scholars date the destruction of Jerusalem by the Babylonians somewhere between 586 B.C. and 590 B.C. Thus in his chronological footnotes in this section of the Book of Mormon, Brother Talmage suggests that the events following Lehi's vision of the destruction of Jerusalem took place sometime after about 588 B.C. (Ludlow, Daniel H. A Companion to your Study of the Book of Mormon. Deseret Book 1976, p. 124.