Because I can't imagine such stories materialize in other places (of developed word and large parts of developing world either).
You just seem to have enough peculiar traits from both gender sides for a perfect storm.
Also the issue here has something to do with bitcoin attracting early adopters and groups with certain political preferences. I certainly wouldn't call the bitcoin subculture mainstream yet.
I think this issue doesn't have to be correlated to "% of women in STEM majors". I think that by tackling this as simple the numbers problem you're only making it worse.
Maybe it's not so much Bitcoin attracting certain early adopters, rather USA having a peculiar supply of early adopters (for many things) that also happen to clash with USA's supply of women? This way you can't win, Bitcoin or no Bitcoin.