What was the impetus for restoring the consecration of virgins for women living in the world in the 20th Century? The United States Association of Consecrated Virgins says the following about their sacramental:

The Consecration of a Virgin is one of the oldest sacramentals in the Church, and one of the fruits of Vatican II was the restoration of this profound blessing on virgins living in the world. The promulgation of this restored Rite for women living in the world was on 31 May 1970.

What was the impetus for restoring the Consecration of a Virgin for women living in the world? As it says, this is an old practice, but seems to have fallen out of favor after a time, so I'm curious as to why it came back.
Ultimately, the impetus for allowing women living in the world to be consecrated virgins was ecumenism and antiquarianism, which influenced almost all of Concilium's liturgical changes after Vatican II.
Pope Pius XII describes antiquarianism as (Mediator Dei):


[…] the restoration of all the ancient rites and ceremonies indiscriminately. The liturgy of the early ages is most certainly worthy of all veneration. But ancient usage must not be esteemed more suitable and proper, either in its own right or in its significance for later times and new situations, on the simple ground that it carries the savor and aroma of antiquity. The more recent liturgical rites likewise deserve reverence and respect. They, too, owe their inspiration to the Holy Spirit, who assists the Church in every age even to the consummation of the world.[Mt. 28:20] They are equally the resources used by the majestic Spouse of Jesus Christ to promote and procure the sanctity of man.



Denying public vows of virginity to women living in the world
Condemnations
Innocent II &quot;condemned as 'pernicious and detestable' [perniciosam et detestabilem] the custom of women wishing to be thought of as nuns, who lived in their own private dwellings rather than in monasteries.&quot;; Second Lateran Council (1139) can. 26 (Stegman 2019 p. 90):

We decree that the pernicious and detestable custom which has spread among some women who, although they live neither according to the rule of blessed Benedict, nor Basil nor Augustine, yet wish to be thought of by everyone as nuns, is to be abolished. For when, living according to the rule in monasteries, they ought to be in church or in the refectory or dormitory in common, they build for themselves their own retreats and private dwelling-places where, under the guise of hospitality, indiscriminately and without any shame they receive guests and secular persons contrary to the sacred canons and good morals. Because everyone who does evil hates the light, these women think that, hidden in the tabernacle of the just, they can conceal themselves from the eyes of the Judge who sees everything; so we prohibit in every way this unrighteous, hateful and disgraceful conduct and forbid it to continue under pain of anathema.

This is similar to reason #4 below.
1927 Holy Office decision
Reflecting &quot;the same response that had been given by the Sacred Congregation in 1597 to the Patriarch of Venice&quot; (Stegman 2019 p. 144), the Holy See in 1927 forbade public vows of virginity apart from entering a religious institute (AAS 19 p. 138):
To the dubium:

An expediat concedere facultatem dandi benedictionem et consecrationem Virginum mulieribus in saeculo viventibusWhether it is expedient [for the pope] to give [bishops] the faculty to bless and consecrate virgins living in the world

the Holy See responded:

Negative et nihil innovetur.Negative and no changes.

Also, &quot;nonnulli locorum Antistites petiissent facultatem [several local bishops requested the faculty].&quot;
Several reasons for the 1927 denial were opined by contemporary theologians, as quoted in &quot;Explanations for the 1927 Denial&quot; (PDF pp. 129-33) of Stegman 2019:

It puts &quot;women in a new sacred state in the Church […] regulated directly by diocesan bishops, and not through the protection of the monastery.&quot;
It &quot;could be likened merely to a private and simple vow, rather than being on par with a solemn vow that would invalidate a subsequent marriage.&quot;
&quot;These virgins must wear special clothes, and especially to continuously wear the veil of consecration; but would the people of our time patiently put up with this when in the midst of society virgins are observed so dressed?&quot;
It &quot;would draw women away from religious life to a life that would be 'far from providing all the means of perfection that religious find in their vows of poverty and obedience, in their enclosure,
in their life in common, in their constitutions and the paternal vigilance of their superiors.'&quot;

Pope Pius XII
Pope Pius XII's 1950 apostolic constitution on nuns Sponsa Christi, art. 3 §3:

§ 3. Solemnes antiquae formulae consecrationis Virginum, quæ in Pontificali Romano habentur, Monialibus reservantur.The ancient formula of the consecration of virgins in the Roman Pontifical is reserved to nuns [of solemn vows].

Consecration of virgins for women living in the world
Before Vatican II, the ceremony for consecrating a virgin,

De Benedictione et Consecratione Virginum(The Liturgy of the Consecration of Virgins according to the Roman Pontifical),

was reserved to women in religious orders.
Vatican II's constitution on the liturgy
Sacrosanctum Concilium:


The rite for the consecration of virgins at present found in the Roman Pontifical is to be revised.


1970 revised rite
Concilium, the group that created the Novus Ordo Mass, Working Group 20b (Cœtus 20bis) implemented this revision. &quot;Not until the sixth and last schema do Consilium archives reveal that the coetus considered the notion of consecrating virgins living in the world&quot;; they also changed the eligibility requirements of a purpose of serving virginity (propositum servandae virginitatis) and carnal integrity (caro integritate) to a &quot;requirement of chastity&quot; (Stegman 2019 p. 121).
Stegman 2019 p. 125:

On May 31, 1970, the Sacred Congregation for Divine Worship, under the authority of Pope Paul VI, decreed the promulgation of the new rite, which provided for the consecration of nuns as well as women living in the world.54 The new rite was to be effective January 6, 1971. “Thus the consecration became accessible no longer to only a few monastic orders (Carthusians and Benedictines) but also to all nuns with solemn vows and to women virgins living in the world.”55

This is in-line with the Vatican II changes that minimize the differences between the laity and religious (cf. &quot;What is the history of the term “state of perfection” before, during, and after Vatican II?&quot;).
The 1983 Code (Can. 604) instated the &quot;order of virgins&quot; according to the 1970 liturgical rite, thus tacitly permitting consecrating virgins living in the world.
In the &quot;spirit of Vatican II&quot; and its love for novelty, the 2018 instruction Ecclesiae Sponsæ Imago on consecrated virgins doesn't even require perfect continence as a prerequisite:

Thus to have kept her body in perfect continence or to have practised the virtue of chastity in an exemplary way, while of great importance with regard to the discernment, are not essential prerequisites in the absence of which admittance to consecration is not possible.

cf. Canonist Ed Peter's commentary
Resources
Public vows of virginity were permitted in very early church history, but since at least 1597, private vows only have been encouraged for women living in the world.
Virginity is a vocation, so perhaps the Church wanted to give more official support to it, considering the drastic decline in female religious life after Vatican II (or perhaps permitting it to women living in the world contributed to the collapse of post-Vatican II female religious life; cf. reason #4 above).*
cf. The Mystery of Love for the Single by Fr. Unger, O.F.M. Cap. (1958) ch. 5 and this informative blog by a consecrated virgin.
*appendix of Liturgical Time Bombs in Vatican II by Michael Davies:&quot;• Sisters. 180,000 sisters were the backbone of the Catholic education and health systems in 1965. In 2002, there were 75,000 sisters, with an average age of 68. By 2020, the number of sisters will drop to 40,000—and of these, only 21,000 will be aged 70 or under. In 1965, 104,000 sisters were teaching, while in 2002 there were only 8,200 teachers.&quot;