Because it’s possible to win the popular vote (one person, one vote) yet still not win the election. Ref: Bush-Gore, 2000; 1876, Hayes-Tilden; 1888, Cleveland-Harrison; 1824, Adams-Clay.

Please note however, that technically the U.S. is NOT a democracy, rather a republic.

An electoral college is a set of electors, who are empowered as a deliberative body to elect a candidate to a particular office. Often these electors represent a different organization or entity with each organization or entity by a particular number of electors or with votes weighted in a particular way. Many times, though, the electors are simply important persons whose wisdom, it is hoped, would provide a better choice than a larger body. The system can ignore the wishes of a general membership whose thinking may not be considered. When applied on a national scale, such as the election of a country’s leader, the popular vote can on occasion run counter to the electoral college’s vote, and for this reason there are some who feel that the system is a distortion of true democracy in a democratic society.