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

Issues of importance to 38 non-battleground states, like Oregon, 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 in 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 support of efforts in swing states.