Why Were Most Voters Left Out of the 2012 and 2016 Presidential Elections?
The fact that the winner-take-all rule has not been challenged in the past is easily explained: the winner of the national popular vote always won the Electoral College between 1888 and 2000.
The reason for the absence of clash between the national popular vote and Electoral college was that between Grover Cleveland's defeat in 1884 (when he won the national popular vote) and the year 2000, the number of battleground or swing states was very large. The non-contestable states were few in number, and fluid from one election to the next. Therefore, campaigns ran national campaigns, guaranteeing that the national popular vote winner would also win the Electoral College.
Only in the current century did densification and polarization produce a situation in which each of the two major parties' candidates chose not to compete in a large number of states conceded to the rival. In 2012, for example Obama did not conduct polls in more than 10 states. In 2016 Trump quite rationally chose to ignore more than 40 states, inevitably lost the popular vote by almost three percent, and won the Electoral College by winning four of the six states that turned on popular margins of less than 2%: Florida, New Hampshire, Pennsylvania, Michigan, Wisconsin and Minnesota.