More accurately, they just stopped making square butt frames (they've been making round butt ones since the dawn of time.)
Well, rather than having to tool up to produce each frame size in two variants, now they only have to do one, which helps lower production costs. Since a round butt can be converted to a square butt feel with a set of grips, while converting a square to a round requires grinding, it really made sense to let the square butt fall by the wayside.