On Oct 3, 1918, Joseph F Smith had this vision. It was dictated to his son, Joseph Fielding Smith, on Oct 6, 1918 (GC was in between, Oct 4 and 5, 1918). The rest of the FP/12 approved of it on Oct 31, 1918. At the Oct 1976 GC, it was canonized first as part of the PoGP, but now is codified among the LDS canon as D&C 138.
By the time of the vision, Lorenzo Snow had been dead for just days shy of 17 years. Joseph F Smith explained that he saw in the spirit world:
53 The Prophet Joseph Smith, and my father, Hyrum Smith, Brigham Young, John Taylor, Wilford Woodruff, and other choice spirits who were reserved to come forth in the fulness of times to take part in laying the foundations of the great latter-day work,
So, Joseph F Smith named each of the prior prophets of this dispensation (and Hyrum Smith too), except Lorenzo Snow. Either Joseph F Smith's file leader in the Mormon hierarchy and immediate predecessor was relegated to merely inclusion in the unnamed "other choice spirits", or Lorenzo Snow was not seen in the vision.
Why was Lorenzo not named?