Because of state-by-state winner-take-all laws, not mentioned, much less endorsed, in the Constitution. . .
In the 2012 presidential election, 1.3 million votes decided the winner in the ten states with the closest margins of victory.
Candidates had no reason to poll, visit, advertise, organize, campaign, or care about the voter concerns in the dozens of states where they were safely ahead or hopelessly behind.
With the end of the primaries, without the National Popular Vote bill in effect, the political relevance of 70% of all Americans was finished for the presidential election.
In the 2016 general election campaign
Over half (57%) of the campaign events were held in just 4 states (Florida, North Carolina, Pennsylvania, and Ohio).
Virtually all (94%) of the campaign events were in just 12 states (containing only 30% of the country’s population).
In the 2012 general election campaign
38 states (including 24 of the 27 smallest states) had no campaign events, and minuscule or no spending for TV ads.
More than 99% of presidential campaign attention (ad spending and visits) was invested on voters in just the only ten competitive states..
Two-thirds (176 of 253) of the general-election campaign events, and a similar fraction of campaign expenditures, were in just four states (Ohio, Florida, Virginia, and Iowa).
Issues of importance to non-battleground states are of so little interest to presidential candidates that they don’t even bother to poll them individually.
Charlie Cook reported in 2004:
“Senior Bush campaign strategist Matthew Dowd pointed out yesterday that the Bush campaign hadn’t taken a national poll in almost two years; instead, it has been polling [the then] 18 battleground states.”
Bush White House Press Secretary Ari Fleischer acknowledging the reality that [then] more than 2/3rds of Americans were ignored in the 2008 presidential campaign, said in the Washington Post on June 21, 2009:
“If people don’t like it, they can move from a safe state to a swing state.”
Over 87% of both Romney and Obama campaign offices were in just the then 12 swing states. The few campaign offices in the 38 remaining states were for fund-raising, volunteer phone calls, and arranging travel to battleground states.