They've gone underground. You just don't see nuns anymore. When did this happen? It seemeed to me right after I left elementary school in the late eighties. In fact the nunnery for my old school is now a Catholic rehab for alcoholics.
We have a Benedictine Convent here in St. Marys (duh?), a few of the dear ladies can be seen driving about town in their pickup trucks.
The 'disappearance' of nuns as we know them can be traced directly, IMHO, to the birth control pill and the feminism that drove it. Not only did it teach selfishness and self-absorption, it also drastically reduced the size of Catholic families (the big Catholic families always had an 'extra' child for the vocations).