One reason "gender expression" is of such elevated importance is because clothes are massively overproduced. So many clothes to choose from and buy. Choosing clothes is now considered a form of "expression" like choosing paints and canvas to make art, or choosing notes and instruments to make music, or choosing words to make literature.
In the past there weren't enough clothes around to be "expressed" with; clothing was scarce, slowly made, intentional, and functional. Clothes have always signaled status, class, and more, but only the very rich possessed sufficient excess for individual "expression".
Now consumers must over-purchase over-produced clothes, for what reason? What justifies this frenzy of overproduced, disposable fashion? Ah, "expression." A cherished principle that must be defended. Go shop for social justice!