The depth of spiritual attainment is determined by the depth of the corrected desire. With each new generation, the souls descending into the world are “coarser” – more egoistic. Therefore, Kabbalists who lived before the Ari (16th Century) had small egos and were therefore able to reveal the elements of creation in full detail, including such concepts as the screen, Tzimtzum Alef, Tzimtzum Bet, Orh Hozer, and so on.
They used these spiritual forces, but they did not completely understand their origin and mode of operation. This is why they could not clearly express it either. It is like seeing a picture: even though you may be able to replicate it, you don’t know how the colors break down into a spectrum, what the wavelength of each color is, and how this affects all of us and turns into feelings.
If one’s desire is not thick enough, then it is impossible to attain the cause of a phenomenon and explain how the Light unites with the desire. The Ari was the first person to reveal the difference and the connection between the Direct Light and the Reflected Light in all its depth. He was able to do this by virtue of having a special soul. In addition, he was living in the beginning of the Messiah’s era, when the souls requiring correction began to be revealed in the world.
Prior to that time, there was a period of exile, when the ego was not revealed in a form ready for correction. This is why it is written that all the Kabbalists before the Ari communicated Kabbalah from the standpoint of the Direct Light (Kabalat Ramak), whereas the Ari explained Kabbalah from the standpoint of the Reflected Light (Kabalat Ari).
Later, Baal HaSulam continued the method of the Ari and made it fit for our use.