Because they need physically strong folks able to fight in subzero temperatures while carrying heavy equipment, willing to run into enemy fire to save their friends or family despite extreme danger to themselves, with familiarity and ability to live in harsh outdoor conditions (or even enjoy it), and with a willingness to bayonet, shoot, or explode a stranger to death because of the uniform they are wearing (with a minimum amount of duress).
They also need folks willing to drive tanks into battle (while going by burning hulks of tanks just like theirs on the side of the road), and handle 100lb shells and fire artillery while being shot at by other artillery, and many other things.
They also need people who can handle being yelled at, mentally and physically abused (by modern standards), and given barely the minimum things necessary to survive - sometimes not even that - and still function enough to survive and even do what they are ordered to do, even if it puts them in harms way. Even if it means doing things no one wants to talk about, or even horrible things.
Statistically, that means men, especially young men.
They did (and probably still do) accept women volunteers. They are out there, but there is not a lot. I can't say I blame them. Some of the videos I've seen, some are holding their own. I definitely respect them for that, they are fighting for what is theirs. It's not a place good for anyone. It is a place some can survive better than others.