Many nuns gave up their habits, moved out of convents, earned higher educational degrees and went to work in the professions and in community service. The study confirms what has long been suspected: that these more modern religious orders are attracting the fewest new members....
"We’ve heard anecdotally that the youngest people coming to religious life are distinctive, and they really are," said Sister Mary Bendyna, executive director of the Center for Applied Research in the Apostolate. "They’re more attracted to a traditional style of religious life, where there is community living, common prayer, having Mass together, praying the Liturgy of the Hours together. They are much more likely to say fidelity to the church is important to them. And they really are looking for communities where members wear habits."
My short explanation of this is that "modern" orders have conformed more to the world and lost their original purpose. If you mainly want to do community service or save the environment, you can do that without vowing celibacy. The point of a religious order is to give your whole life to God, not to any lesser cause.
A more difficult question is how the heck so many orders lost their focus in the first place. You could write a whole book about it and I'd probably still wonder, but here's a long interview about the shockingly swift collapse of one order of nuns.
We inundated that system with humanistic psychology.... There were some 615 nuns when we began. Within a year after our first interventions, 300 of them were petitioning Rome to get out of their vows. They did not want to be under anyone's authority, except the authority of their imperial inner selves.