Lehi's family left Jerusalem through direct revelation from the Lord. Nevertheless, there was significant dissension in the family, notwithstanding the direct command of the Lord that they go. With Ishmael, we hear of no direct revelation. We know that Nephi and his brothers "did speak unto him the words of the Lord" (1 Nephi 7:4) but that is quite different from having the Lord speak directly. Ishmael is perhaps a prototype of the many who would later be converted to the gospel through the message of the missionaries. Ishmael receives the word of the Lord through His emissaries. Ishmael's faith was sufficient upon hearing those words (and clearly receiving a testimony of their truthfulness) that he uprooted his entire family and headed off to an undescribed land of promise.
It is not simply revelation which has the power to move, but the word of the Lord communicated through the spirit. Many of the early saints of this dispensation have followed in Ishmael's lead - hearing the word, knowing of its truth, and giving all to follow it.