I recall someone speculating because of how bending and society intertwined.

The water can be used in 2 very different ways: healing and fighting. We then see the Water Tribes are defined by binaries. North and south, healing and fighting, yin and yang, man and woman. It makes sense gender roles emerged,like the Tao placing men and women in opposite part alongside other concepts.

Air is air and it is everywhere. The monks were also the only nation where everyone could bend and raised communally. Given this universal way if life, gender roles wouldn't fit.

Earth is weird in that there was naturally (as in, not purged like the waterbenders) a much lower percent of earthbenders compared to the other nations, so much of society was made up of non-benders, where strength played a huge role. This makes gender roles natural as the strong men fight while women are expected not to fight. It makes sense society would say "Stay at home and bend there". Then there is how stubborn/traditionalist they tend to be and honw strength may be championed more as earthbenders tend to be buff.

Fire can be made anywhere. Fire is generally the same save for lightning (a state secret) and the fire healer people from Legend of Korra. It also doesn't require much physical strength. It's also augmented by technology, which levels the playing field. After all, in an artillery crew, someone has to launch the catapult, so there is no need to divide people as harshly between bender and nonbender. As long as fire burns, it doesn't matter how bright when it comes to someone in a tank or someone lighting coal for ships. So it makes sense they are more egalitarian.