You'll have to forgive me for finding this non-explanation unsatisfying. Do you think there's any reason for it being "rare"?
What went wrong - in fubar situations, men instinctively lunged for protecting women, instead of rationally estimating situation and acting accordingly. We men are simply still too much gentlemen to have women around when bullets are flying, despite feminists trying hard erasing this. Give it 2 more generations and western society will be there.
FYI eastern Europe countries like Ukraine have women in the military, including combat positions. Not surprisingly they keep getting injured and dying just like rest of them.
This report might be useful. It summarizes a large scale study done in 2002 by the British army. The tests were heavily rigged in favour of the women but even so the conclusion was to keep them out of combat roles. Note the part where they say that fewer than 2% of women were as fit as the average male soldier:
https://www.cna.org/reports/2012/Practices-of-Foreign%20Mili...
A panel of subject matter experts conducted the study. They issued a report, A Study of Combat Effectiveness and Gender, to British ministers in 2001.[24] The study's tests were designed to examine the feasibility of mixed-gender tank crews, all-women crews, mixed infantry units, and all-women infantry units. They also were designed to examine how men would react to the presence of women on the battlefield and how each gender coped with the physical demands of combat.
According to news articles, some reports maintain that the exercises found that women were as capable as men for service in combat units, but the results were mired in controversy [56]. Senior military officers, including Brig Seymour Monro (the Army's director of the infantry), stated that the Army field tests were so diluted that they “amounted to little more than aggressive camping.” Brig Monro also said that tasks that women were not physically capable of doing were simply dropped from the trials [56]. According to the final Ministry of Defence report, the study showed that fewer than 2 percent of female soldiers were as fit as the average male soldier [57].
Specifically, news reports stated that the trials stalled early on when women were not able to complete a number of tasks under battlefield conditions:
• When asked to carry 90 pounds of artillery shells over measured distances, women failed 70 percent of the time (compared with a male failure rate of 20 percent).
• When asked to march 12.5 miles carrying 60 pounds of equipment followed by target practice in simulated wartime conditions, women failed 48 percent of the time (compared with a male failure rate of 17 percent).
• Women were generally incapable of digging themselves into hard ground under fire.
• Women were generally slower in simulated combat exercises involving "fire and move" drills.
• Women suffered much higher injury rates in close-quarter battle tests, such as hand-to-hand combat.
It might be interesting to think about what sets them apart from regular armies, which have often had trouble in this area.
You're confusing the requirements and needs with regular armies.
No, women have been excluded from front-line combat because they are physically weaker than men and would be killed quickly without accomplishing much. Men are just much more aggressive, have higher stamina etc. It's never been about repopulation. Nothing biologically stops a woman having 10 children with a single man, it's just rare.