How much do you hear at all about plumbing or other trades? A few minutes on Google turns up links like http://womeninconstructionconference.com/, http://www.new-nyc.org/, http://www.explorethetrades.org/women-in-trades/, etc. so I think it's most likely that you're seeing a combination of selection bias and the fact that software development is both a popular field getting general attention and one with an abnormal diversity rates compared with similar fields.

That's not saying that those fields are perfect but, say, a female/Hispanic/etc. accountant, paralegal, etc. won't be the only person fitting that description at most meetings. Skilled trades often have skewed gender, ethnicity, etc. ratios but they also don't get anything like as much attention so most of us are unaware of the problem or efforts to improve things.