Kelly Dittmar. "Women in the 114th Congress",Posted on 1/6/ 2015, http://cawp.rutgers.edu/footnotes/category/congress
 Daniel Pouzzner. "The Evolutionary Psychology of Human Sex and Gender," 2000-Apr-11/12, http://www.mega.nu/gender.html
 Rhea E. Steinpreis, Katie A. Anders, and Dawn Ritzke. University of Wisconsin-Milwaukee. "The Impact of Gender on the Review of theCurricula Vitae of Job Applicants and Tenure, Candidates: A National Empirical Study", 1999, http://advance.cornell.edu/documents/ImpactofGender.pdf
 Claudia Goldin, Cecilia Rouse. "Orchestrating Impartiality: The Impact of "Blind" Auditions on Female Musicians", NBER Working Paper No. 5903, Issued in January 1997, http://www.nber.org/papers/w5903
 "Male–female income disparity in the United States", http://en.wikipedia.org/wiki/Male%E2%80%93female_income_disparity_in_the_United_States
 "Employment Characteristics of Families Summary", Economic News Release, http://www.bls.gov/news.release/famee.nr0.htm
There are many reasons.
In the U.S., the political campaign atmosphere has turned nasty and is distasteful to most women.  Respect for the individual candidate and the individual's family needs to be re-introduced to the process.
Big money equals election results, and the big money machines are financed primarily by white men.
We humans have a part of our brain, the basal ganglia, that deals with ancient survival instincts. As voters of any gender, this causes us to have a bias toward choosing a tall, strong, handsome man to physically slay the saber toothed tigers and repopulate the tribe with those genes, rather than choosing someone who is empathetic, collaborative, and cooperative. 
Multiple studies have found direct evidence of (unconscious) gender discrimination by both men and women. For example, in a study using identical resumes, there were fewer replies to the resumes with female names.  Another study showed that when orchestra auditions were changed to be blind auditions, that is, the performer was not visible, only the performance was heard, more jobs went to women. 
After the 2014 election, women comprised less than 20% of Congress, the most in its history.  After the 2016 election, the U.S. made zero progress in adding women to Congress.
For decades, there have been many organizations making heroic attempts to increase the number of women in Congress. But, at the current rate, it will be many centuries before gender equality is achieved,  and there are reasons to suggest we will never get there.
One need only look at the Forbes list for the 50 wealthiest Americans published in 2014 to notice that out of the 50, only 6 are women.  The "early money system" (significant campaign funding very early in a race) tends to favor men. 
Again status quo, you tend to support (give money to) people for political positions who are a reflection of yourself.
There is a significant gender difference in personal income. In a large study, of the respondents who had “considered” running for office, 56% of those men, but only 41% of those women had annual incomes exceeding $100,000.  Per the U.S. Census Bureau, in 2010 the median income of full-time year-round workers was $42,800 for men, compared to $34,700 for women. 
less likely to receive support from political parties. 
more likely to encounter roadblocks when they try to plug into networks for successful fundraising. 
less likely than men to receive the suggestion and encouragement to run for office. 
Studies show women are still responsible for the majority of childcare and household tasks.   And 70% of mothers with minor children are working or looking for work.  For mothers who need to work outside the home to provide income, this leaves no time for the extracurricular tasks of networking, fundraising and campaigning.
And these are only some of the challenges that women face.