The American Civil War was a conflict fought by MEN. Although there are a few rare examples of women who disguised themselves as men so they could fight, the woman's role on the battlefield was practically non-existent. Some wives did follow their men (sometimes travelling in wagons or on horseback)when units moved out, but they were restricted to far beyond the battle lines. Prostitutes also emerged during the war, following many Union armies to ply their trades during the soldiers' off-hours, but they were also not present when action was expected. Crane's story tells of a bloody battle and the reactions of a unit of first-time soldiers engaging in their first action. There is little place for women in this scenario, so Crane wisely left them out of the narrative.
Sadly, most modern writers would include women for the romantic angle that could be developed. Of course, women play a much larger part of active military units throughout the world today than during Crane's time.
During the time of the Civil War women were expected to stay home and care for the farms and the children. They had specific patterns established and the concept of a woman serving in war was unfathomable.
The novel by Stephen Crane was developed more as a boy's experience and the role of women in the novel is that of the nurturer and homemaker who depends on her son but also knows she has to let him go. The novel is about a growing boy who is maturing while coming to terms with war.
A writer in this day may still leave women out of the war story if the focus is on one particular person or focus. However, having women serving in the war effort now and leaving them out might make the novel less accurate or believable.