My Answer: It is impossible to know why the souls were arranged in this manner from Above. Baal HaSulam wrote that he did not know why he had merited such a great mission. However, we need to understand that only our corporeal perspective sees certain souls as extraordinary and others as unimportant or “secondary” ones because we see through the prism of our egoism.
However, if we are talking about the spiritual world, then there are no “great” or “small” souls there. Everything is measured in bestowal, and therefore the greatest one is the one that bestows the most, the one who serves everyone, and is situated below everyone else serving as support and a foundation for the others.
From the 4th part of the Daily Kabbalah Lesson 7/15/10, Pri Hacham, Igrot (Letters), Letter 39