[mhays]

Often they were specifically a reaction to the mass protests in the 60s etc.... race riots, antiwar, etc. A lot of developers, government clients, and even retailers were fearful of windows. (This was certainly one of the factors in the mall trend too.) Even a lot of downtown condo residents wanted to be in fortresses. (Another of those factors that aren't "like" but were important to clients.)

I'm trying to remember the last brutalist building that works as a retail center or plaza...

Actually I like some brutalist public space, like Seattle's Freeway Park (if that counts), including its central waterfall which is a jumble of board-formed concrete boxes with a great stair path through it. But largely the plaza surrounded by bunker is a brutal failure time and again. Trees can offset that somewhat, but even then. In particular, a blank plaza next to yesterday's fad architecture is a terrible recipe for attracting people.