Because She is the ‘binding’ of all the sephirot. For if the sephirot would ‘pour’ outside of Her, there would be ‘separation’, and thus, there is no permission for a sephirah to pour to any place – except from Her – towards those below.
And thus, is stated of Her: (Jer. 9:22) Let not the wise man boast of his wisdom... etc.... except in ‘this’ (zot).
(Lev. 16:3) In this (zot) manner shall Aaron approach the sacred... For no prophet or sage has permission, to know anything knowable above, except through Her.☉Malkhut