About Keter, the first foundation, it is forbidden to ask. From Chochmah onwards it is a mitzvah to investigate.
Keter is bound up with Eyn Sof — its why is forbidden ground. From Chochmah onwards, investigation is required.
Op. 14 named Keter as the architectonic foundation in which the reason for the whole is embedded. Op. 15 draws the line. Keter is the threshold of investigation. About Keter we do not ask; from Chochmah onwards, we are commanded to investigate. The chapter sets the epistemic discipline that runs through the rest of the book.
The Op. 1 Will/Essence restriction is the deepest rule. About God's Essence we do not speak; about His Will we may speak with appropriate care. Op. 15 extends the same discipline structurally. Keter — the highest of the ten Sefirot — is so closely bound up with Eyn Sof that asking why Keter is what it is would be tantamount to asking what Eyn Sof is in Himself. The restriction holds.
From Chochmah onwards (Chochmah is the second Sefirah), the situation changes. Now we are dealing with revealed structures rather than the foundational ground itself. About Chochmah, Binah, the seven lower Sefirot, the Partzufim, the operational coupling — about all of these, investigation is a mitzvah, a positive religious obligation. The cosmic structure He revealed is meant to be understood; the more deeply, the better.
This is the doctrinal license for the entire rest of Klach. The Tzimtzum unit, the Adam Kadmon unit, the World of Nekudim, the World of Repair, the named Partzufim, the closing Coupling-order — all of this is from Chochmah onwards. The investigation Op. 15 commands is the investigation the rest of the book performs.