Based on the electoral college system, all a candidate has to do is win the swing states (Florida, Colorado, Virginia, Ohio). There are states that will always go red (ex. South Carolina, Texas) and states that will always go blue (California).
That's the case with any undemocratic system - we have the same thing with constituencies in the UK, where most of the country have no say in the election.
In the case of the US election, where you're electing a single person, there's no reason other than subversion of democracy to not have the popular vote decide the election. It's still a bad system with results tending to a two-party system, an AV system on the top would help with that.
Not that democracy is the be-all and end-all, 51% of a country can oppress 49% (or in the case of the uk about 35% of the electorate)
The US is trying to implement a "one-man one-vote" system, where everyone's vote is of equal value - with the National Popular Vote Interstate Compact. It would detract from the political emphasis on the swing states though, you won't catch Ohio signing up for example.
But such a move wouldn't change anything, only once in living memory has the presidency gone to the candidate who lost the popular vote. That vote was possibly the most important presidential vote in recent history, Bush vs Gore.
However america has been carefully cultivated to spilt into two equal sized identical camps, with half the country blaming their woes on the republicans, and half on the democrats, and not realising that both sides are pretty much indistinguishable.