Because when women speak up and voice their opinions, men feel entitled to do things like this (serious trigger warnings).
The threat of violence, particularly sexualized violence, is a favorite tool for insecure men, usually safely anonymous themselves, for trying to push women out of the public sphere. Women targeted by AutoAdmit found themselves wondering which of the men they knew were posting updates on their locations and making rape threats; last year, Jill missed several days of classes. Melissa McEwan dropped out of the Edwards campaign after receiving increasingly credible threats. Amanda Marcotte’s been threatened. Pretty much every feminist blogger has, at one time or another — and you should see some of the stuff that turns up in our moderation queue.
I encourage you to check out the rest of the post!