Because society is ever-changing and accepting more and more everyday.
Indeed, things are bound to change over time,
Anyway, when I first saw the topic title, I was going to say it's probably in a large part due to pop culture and fashion. The images that models, pop musicians, and such maintain might differ from what was seen as ideal in previous generations, and of course, people who grow up being exposed to this
However, since it seems to be a matter of gender roles rather than appearances that this topic is about, I would say that yes, it can be attributed to the changes of society, specifically values in regards to how either six should behave and their roles in society. In the past, different things were expected from men and women, and they had different roles in society, they would also recieve different treatments and women did not have the same oportunities as men, but this sort of thing would naturally change if society's values change, if society moves towards sixual equality, and compared to how things were a hundred years ago, it has certainly moved in that direction, at least in some parts of the world, it seems to me that with it, it would only be natural that what would be considered "acceptable" for men and women would also change. And just as more "feminine" men might become increasingly common, the same goes for women who don't fit the "traditional" expectations of their six, in fact, I might say that this is even more true for the female six, as traditional society usually put men in a more dominant role, and usually, movements aimed at gender equality seem to be focused around promoting women's rights, thus I would expect society's standards towards women to see the most change.
Of course, if by "manly" you mean "taking responsibility", then that's a whole other subject for discussion, and honestly, it's something that has nothing to do with gender roles. Whether male or female, people should fulfill their responsibilities, and this was expected of them in the past too, changing gender roles means the responsibilities either six is expected to fulfill change, but that doesn't change the fact that, when people have a responsibility placed on them, they're expected to fulfill them.