Why do less women backpack than men?
A few thoughts:
Socialization plays a huge roll. If not exposed to outdoor activities early, we are far less likely to internalize the "role" or view of ourselves as "someone who does those things". Sure, some people are late adopters, but they are the exception, rather than the rule. For example, what percent of women in Duluth, MN do you think would describe themselves as "campers" or "hikers? While I don't have hard data here, I'd be willing to bet it's considerably higher than the percentage of men in, say, Houston, TX.
Second, while I don't buy the "caveman theory" as detailed above, there are several established biological/evolutionary factors that account for at least some of the variation in outdoor activities between sexes. Throughout most of human evolutionary history men have been hunters, venturing out from settlements and navigating in the wild, whereas women have been limited in range to tend to the young.
A resulting physiological difference is in how sexes brains handle navigation. The male brain tends to navigate via an "internal compass" mechanism. If you put a male somewhere and have him move about, he will usually have a mental map that he imagine himself moving through. This contrasts how women's brains tend to handle navigation – through reference to landmarks and objects. Thus, men are better at navigating through unfamiliar terrain – such as when chasing an antelope around a valley days from "home", whereas women are better at navigating through familiar terrain, such as a village or known valley. You may notice this difference when asking for directions – men will tend to say "turn east onto Franklin Avenue" whereas women would tend to say "turn right at the gas station".
While individually not huge, subtle differences like these can and do contribute to one's comfort with (propensity to engage in) activities like backpacking.
Just pieces of the puzzle, I'm sure, but ones worth thinking about.
