Because women's calves are positioned lower than men's. Women's bindings are designed with that in mind, so the highback doesn't cut into calf.
Why not look into men's bindings? You don't want soft, and you don't want pink, and those are pretty much the two defining characteristics of women's bindings.
Tried to look up what makes woman's bindings "woman specific". Found some aspects: Narrower, more padding, different flex (softer?), different highback (lower?) and colors (rainbows and unicorns
Now, my boots are rather bulky, have to sqeeze them into the M size Fames (length is perfect). They could be wide enough to also fit S men's bindings well
Also checked the hight of the boot with hunbands K2 Company M which have a rather high highback, and the rim of the boots is app. 1.5cm higher.
Any woman here riding with men's ones? Or do you guys know of women riding with mens and feel comfortable? Seen many reviews on shayboarder on men's ones, thus I assume, the different position of calves is not always an issue?