I don't know the reason and I don't care to guess. The same applies to sciences as well. It's certainly not limited to CS or IT.
Some people think it might be worth to sacrifice a little so things might get a bit better in the long run, and that every small step counts to be welcoming to anyone.
Edit: Note that I ask for either or in the first paragraph because I believe the latter would be a more easily fixable scenario than the former.
I _think_ it's both. Certainly it is plausible that female and male brains are wired at least slightly differently. But there must be some social factors is play as well.