Why did Adi Shankaracharya say “Nari Narak ka Dwaar”?

Nari, the female element, is the ultimate origin of the science of becoming. The
female element takes different forms and by taking those multiple forms shapes
the varying realities of the male element. A male becomes son because a
female decides to be a mother. A male becomes husband because a female
decides to be a wife. A male becomes father becomes a female decides to be a
daughter.
Without a daughter, a male is potential father. A potential father is perfect father
because there is no daughter to make him conscious of his weaknesses and to
help him correct those weaknesses for realizing perfection as a greeter of life and
be a perfect greeter.
All hell breaks lose whenever the female element makes the male element
conscious of the potential for perfection in an ascending-order. The male wishes
to enjoy the heaven he has created through his efforts to date. The female
wishes him to enjoy the potential heaven he is capable of creating with goal-
oriented focus to his present efforts.

Therefore, Adi Shankracharya said that the female element is the door to hell.
She does not let one live in a state of inertial stupor. She is always moving fast,
and therefore immovable (sthavara).