Baal HaSulam was rewarded with very high spiritual attainment. However, he was pleading with the Creator to lower him from his spiritual level in order to pass the wisdom of Kabbalah down to the people. The most profound factor that helped him to accomplish this was the inclusion of the ARI’s soul (Ibur Neshama) in him. The entire method of the wisdom of Kabbalah regarding the last stage of correction descends to us from the ARI. It explains how to exit the fourth and last exile to the complete spiritual redemption.
Therefore, as soon as the ARI’s method was revealed, no true Kabbalist studied any other books written before the ARI. In response to his request, Baal HaSulam was granted by the Creator the revelation of the soul of the ARI within him. This is what allowed him to write the Sulam Commentary on The Book of Zohar and The Study of Ten Sefirot, the commentary on all of the ARI’s works that describe the structure of the spiritual world.
No Kabbalist that came before him was able to describe the method of the “three lines” of the soul’s correction. As it stands, the entire Torah is the middle line; mutual guarantee is the middle line. Our major work and the revelation of the Creator to us lie in the middle line. Hence, exposing the method of working in the middle line is spiritual salvation for us.
There is no Kabbalist to match the ARI who discovered this method, and there is no Kabbalist to match Baal HaSulam who passed it down and explained to us how to work with it. This is why his books are the source of Light for us.
From the 4th part of the Daily Kabbalah Lesson 9/16/10, Yahrtzeit Baal HaSulam