I would argue that the Equal Rights Amendment (ERA) failed to be ratified because of an effective campaign against it that stirred up fears that the amendment would be used in ways that very few people of the early 1970s would have been able to accept. The ERA’s opponents managed to convince people that the amendment would be used in all sorts of ways other than to end discrimination against women.
For example, the opponents of the ERA argued that the amendment would have required women to be drafted into the military since men could be drafted. The opponents argued that sexual assault laws would be unconstitutional as would laws that provided alimony for divorced women. Opponents even argued that bathrooms that were segregated by sex would become illegal just as bathrooms that were segregated by race had. Opponents of the ERA convinced enough people that the amendment would have these effects that they were able to defeat the amendment and prevent its ratification.
They didn't ratify it because most women did not want it to pass.