This article is from the Alien Movies FAQ, by Darryll Hobsonand Eelko de Vos E.W.C.deVos@TWI.TUDelft.NL with numerous contributions by others.
- The alien was coming to the end of its life cycle, when Ripley happened
to disturb it. It was slow to attack because it was dying. This theory is
supported by an older version of the "ALIEN" script where Ash reveals that
the alien had made a nest and ensured the continutation of its species
(cocooned Dallas and transformed Brett into an egg) at which time the alien
itself would approach the end of its lifecycle; curl up and die.
- The DH comics speculate, that the Aliens are more prone to attack, when
(somehow) threatened. Since Ripley pretty much is defenseless, can't escape
and isn't attacking, why should the Alien hurry?
 
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