Army Chicks are doing it for themselves,

 In a whimsical twist of fate, it turns out that the gates of hell had a major infrastructure update planned during the same period when women were granted the opportunity to join the military in combat roles. The demon contractors were all on vacation, sipping lava cocktails in a tropical underworld resort, leaving the gates unattended. As the paperwork for women's inclusion in combat roles went through bureaucratic circles, the demon foreman looked at the schedule and said, "Oh, we can't do any major soul-consuming events during the renovation. We'll have to reschedule for a later date."

So, while some might attribute the lack of apocalyptic events to societal progress and gender equality, the real reason the gates of hell didn't open was simply bad timing for the underworld renovation crew. Hell hath no fury like a demon foreman forced to reschedule.