Because of state-by-state winner-take-all laws, not mentioned, much less endorsed, in the Constitution. . . 

Wisconsin Gov. Scott Walker in 2015 was correct when he said
"The nation as a whole is not going to elect the next president,"
“The presidential election will not be decided by all states, but rather just 12 of them.

Candidates have no reason to poll, visit, advertise, organize, campaign, or care about the voter concerns in the dozens of states where they are 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).