From The Pillar:
In 1965, convents were booming with vocations, with 181,421 religious sisters in the United States. Five years later, those numbers had plummeted with only 153,645 women religious, as thousands of women left the convent and fewer women entered.
The Second Vatican Council brought major changes to religious life as communities instituted new changes to their structure and mission. Coupled with the sexual revolution and broader cultural changes, and the well of vocations seemingly dried up, with fewer and fewer women joining religious communities. Over the four decades following the council, there was a 66% decline in women religious, and now there are roughly 40,000 religious sisters in the United States, according to researchers.