What they need right now is infantry type fighters.
Physical strength is a factor.
They haven't got the time, the inclination or the resources to test if people are strong and fit enough, so are going with the population-level likelihood.
There is no reason not to deploy women in many other roles, but if that means higher-tech warfare, you don't have the time to train up conscripts. If you're re-roling people, it'll be from your standing army/navy/airforce, and will backfill their less skilled jobs with conscripts. Or it'll be this early with matching skills from the civvy sector.
There's no particular reason not to conscript women into logistics roles, defence medical roles and various others. We did it to an extent - land girls and munitions workers. The whole of the remaining population is likely good no to find themselves in a role that supports the war effort in one way or another, unless utterly incapable.
But conscription means infanteers and, absent the ability to select for strength or other aptitude, you have no choice but to go for the demographic that is most likely to be strong enough.
Also, the patriarchy. If you look at the Ukrainian military, it's heavily male dominated - though now about 10% women. IIRC, there were next to no Ukrainian female soldiers on peacekeeping ops in the 90/00s - compare the Scandinavians where they had women tankies crewing APCs. I think we would see quite different patterns of conscription in countries with higher levels of female participation in the military.