I think it's more just that it's the most obvious aspect to focus on as part of the news. Even when you have well-known women being put into traditionally male roles, it seems to be the fact that they're female that becomes the talking point.
See this from a year ago:
(And with that example, they can't even use the excuse that it was the first time.)
Whittaker isn't a particularly well-known actress but even with a bigger name I think we'd see the initial reporting focus on the gender-swap.
I look forward to the point where that kind of thing is not such a big story any more.