Because it's a day to call attention to sexism against women. And there needs to be such a day, because the vast majority of the time, when something sexist happens, it's against women.<br />
Think about women around the glob. Think about your own society, even! Without even knowing what country you live in, I am 100% confidant there is more sexism against women than men. Probably WAY more sexism against women than men.<br />
And frankly, if you look at it from a perspective of what the day was intended to be (a day to call attention to the issue of sexism against women) and you decide it really isn't that necessary, I can only assume you are either particularly unintelligent or you really haven't ever critically analyzed the world in which you live.
What difference does it make? Both would be meaningless.
We (men) get a whole month for International Mastur*atio* Month. Just don't forget to celebrate it.
because all the other days are mens day
and no international scarf day either :(