I've learnt better than to say I have evidence for this, but I think gender roles originate in differences, psychological and physical, between males and females, and therefore that they are not 'needed', shouldn't be enforced, and will exist without encouragement from society. I can't prove this of course, just as those who say they are purely social constructs and without them the interests of different genders would be much more diverse cannot prove that (as far as I know). I think there is a lot less pressure in modern society, even to the point where parents don't want their child exposed to 'typical' gender stereotypes (like dolls and pink for girls, cars and blue for boys), to conform to gender roles. As far as I know, gender is not much of a factor at all in determining whether you can study in your preferred field and become employed (unless it's construction or something like that). And yet more females still study the arts rather than pursue jobs in mining or science.
And other than that, basically I think it's because women sort of had their femeninity forced upon them and treated unfairly/given special treatment that they are allowed to do typically male things now. It's empowering because males were dominant in ages past. Which is kind of sexist, because when I think about it it's like I've said "Girls can do it because if you say anything about it, that means you're enforcing gender roles which is taboo because they're women." Trousers are also more practical.
tl;dr: I have no evidence for anything. Good day to you.