When the U.S. Constitution was written in 1787:
U.S. population was less than 4 million people  (compared to 320 million today). 
An estimated 20% of the population in the original 13 States were slaves. 
The founding fathers drafted the foundation for a new government but they were limited by their own homogeneous perspective and that of their powerful land-owning peers – all of whom were white, wealthy and male. 
To get agreement from all the States’ representatives, who wanted to keep power in the hands of land-owning white men, they were limited in the scope of changes they could make at that time. They likely felt they needed to keep the initial document short and vanilla for version 1.0.
Geography (representation from each of the 13 states) was the designated basis for representation.
"Representation" was limited to white men (who comprised less than 40% of the population).
Women and non-white men were not allowed to vote - leaving the majority of the nations’ population disenfranchised.
Even though the Constitution starts with “We the People of the United States, in Order to form a more perfect Union," the “People” referred to less than 40% of the American population.
After 1787, the new nation evolved. And in 1870, after the civil war, the fifteenth Amendment was ratified to recognize all adult males as citizens having the right to vote. Women were still excluded from having a voice in how their country was run, and were not allowed to vote. So now just under 50% of the adult population were allowed to vote.
After 1870, the nation evolved and fifty years later, in 1920, the nineteenth Amendment was ratified to allow women the right to vote. Finally, this granted basically 100% of the adult citizens the same voting rights.
And now, we are nearing the 100th anniversary of women's right to vote. But women, who are a majority of the population, still have less than 20% representation in Congress after the 2016 election.
 “Women in National Legislatures: Rankings and Election Systems”, 2/6/15, http://www.representation2020.com/women-in-parliaments.html
 http://en.wikipedia.org/wiki/List_of_U.S._states_by_historical_population, 2/8/15
 http://en.wikipedia.org/wiki/Demographics_of_the_United_States, 2/8/15
 "Founding Fathers of the United States" 2/6/15, http://en.wikipedia.org/wiki/Founding_Fathers_of_the_United_States