You can blame the birth control pill for that. I wouldn’t place too much faith In That hypothesis.
If a square jaw at least subconsciously signals a man who is physically fit/strong, then I think there is an argument to be made that as physical strength becomes less important in evaluating a man’s attractiveness as a mate, women will gravitate more toward men with softer features. I think of my close group of female friends from college, most of them ended up marrying nice, smart guys who went on to make a lot of money, not necessarily traditionally handsome guys.
On the birth control thing, I know there have been studies indicating a link beteeen a woman being on hormonal birth control and her mate selection, but I’d be interested in seeing data around what percentage of women are already on the pill when they meet the father of their children. Among the women I know it’s pretty split. When I met my husband I wasn’t on the pill and he’s the strong-jawed type, but I also consciously was going for that having a weak jaw myself.