There’s a 50/50 split among the industrial designers at my company (but there’s only four of us and they just hired us two women this year, so we may not be a good sample).
I think it’s mostly personality, and ID people have more of a tough “manly” personality than other design fields. From what I’ve experienced in school and in the real world, successful female industrial designers seem to be more of the “tomboy” type who aren’t afraid to get dirty in a machine shop making prototypes who have a tough outer shell holding their ground defending their design decisions and remaining composed and open to criticism when their designs get torn to shreds in critiques. This type is a minority of women in general, and then you take a fraction of that minority that decides to go into ID and that’s why there are fewer women in ID, in my opinion.