Because it was fashionable to do so in the 1970s ... and they bought in to the satanic lie that by ditching their distinctive garb, their uniform, they would be seen as "relevant" by the world, and their "message" (whatever it may be) would be more easily accepted.
Instead, they signed their organizational death warrant.
It's not a difficult concept ... lots of folks understand it.
Indeed. Out little church school was run by three nuns from up North. In the early 60s, the sister superior confided in me that her order was thinking about discarding their habits, but that if they did, she would leave. Habits creates an esprit dcorps by subordinating physical appearances, One troop in a platoon looks more or less like another until you get up close. Habits also women from immodest glances if they happen to be pretty or regarded with disdain if they are homely. Even homely women tend to look better if one sees only their faces.