Because studies have shown that people who think they are evaluating conference papers (or pretty much anything else ever) on technical merit alone often are not doing anything of the sort, and are reinforcing their existing gender biases through their selection process. Two quick examples off the top of my head: - http://www.princeton.edu/main/news/archive/A94/90/73G00/ - http://www.pnas.org/content/early/2012/09/14/1211286109#aff-...

It's not some kind of man-hating reverse discrimination, it's an attempt to correct very real and very pervasive institutional inequalities.