According to a new study of how parties recruit candidates to run for office, women just don't get asked as often as men:
They are less likely than men to be recruited intensely. And they are less likely than men to be recruited by multiple sources....These findings are critically important because women’s recruitment disadvantage depresses their political ambition and ultimately hinders their emergence as candidates.
This comes via John Sides at the Monkey Cage. The chart below shows the basic findings. To be honest, I'm surprised the numbers are as close as they are. I would have expected an even greater disparity.