The Perennial Philosophy presents a universalist interpretation of the world's religions through a comparative study of a collection of mystical writings, from both Eastern and Western philosophies. Although I have a brief understanding of the Zohar, I noticed remarkable similarities. So this led me to question the complete exclusion of the Zohar and Kabbalah from the book. Is it possible that the Zohar wasn't yet translated at the time and knowledge of Kabbalah wasn't popular?
Whatever the reason was it had nothing to do with availability of translations, see Huss's survey Translations of the Zohar: Historical Contexts and Ideological Frameworks. Large parts of Zohar appeared in Latin in the second volume of the highly infuential book Kabbalah Denudata (1684) by the Christian Kabbalist Knorr von Rosenroth. First English translation of excerpts from Zohar, based on Rosenroth, was published in London in 1887 by Samuel Liddel MacGregor Mathers, an English occultist and one of the founders of the Hermetic Order of the Golden Dawn. In 1888 US freemason Isaac Myer published a translation from the original Aramaic in Philadelphia. In the early twentieth century English translations by Nurho de Manhar (a.k.a. William Williams), also a member of The Golden Dawn, were published in the occult journal The Word. The first comprehensive French translation, by Jean de Pauly (presumably, the converted Jew, Paul Meyer), was printed in Paris by Emile Lafuma in 1906–1912. This translation attracted much attention and made Zohar well known in Europe and the US.
By 1940-s, Huxley's The Perennial Philosophy came out in 1945, there were already academic translations and commentary on Zohar available. The most notable work was due to Jewish Kabbalah scholar Gershom Scholem, namely the 1926 article Did Rabbi Moses de Leon write the Zohar, and two chapters in his 1941 book Major Trends in Jewish Mysticism, where he concluded that Zohar was a pseudo-epigraphic work written by Moses de Leon. Scholem's ambivalence about the text influenced many subsequent scholars, Orthodox Jews generally view Zohar as apocryphal. Scholem wrote in 1941:
"To the streak of adventurousness which was in Moses de Leon, no less than to his genius, we owe one of the most remarkable works of Jewish literature... The author’s spiritual life is centred as it were in a more archaic layer of the mind. Again and again one is struck by the simultaneous presence of crudely primitive modes of thought and feeling and of ideas whose profound contemplative mysticism is transparent... a very remarkable personality in whom as in so many mystics, profound and naive modes of thought existed side by side."
Perhaps this influenced Huxley's decision, but this is merely speculation. Still, it does not fit very well with his focus on exalted "saints and sages" in The Perennial Philosophy:
"If one is not oneself a sage or saint, the best thing one can do, in the field of metaphysics, is to study the works of those who were, and who, because they had modified their merely human mode of being, were capable of a more than merely human kind and amount of knowledge."