US Constitution Article 2, Section 1
The executive Power shall be vested in a President of the United States of America. He shall hold his Office during the Term of four Years, and, together with the Vice-President chosen for the same Term, be elected, as follows:
Each State shall appoint, in such Manner as the Legislature thereof may direct, a Number of Electors, equal to the whole Number of Senators and Representatives to which the State may be entitled in the Congress: but no Senator or Representative, or Person holding an Office of Trust or Profit under the United States, shall be appointed an Elector.
At first, people could not vote for US Senators. Instead, each Senator was chosen by his state legislature. After a while, that method was considered to be too undemocratic. So it was changed in 1913 with the 17th Amendment.
Yet today, you still cannot vote for president. Instead, you vote for a bunch of guys who will choose a president for you. It works well enough most of the time, but every now and then the people choose one candidate and the electorial college chooses a different candidate.
Some argue that the electorial college promotes state's rights and federalism. But under a popular vote system, states would still retain the right to control who gets on the ballot. A popular vote selection process would give more voice to individual voters while still recognizing the role of the states.
Would Gore have won in 2000 if the race were determined by popular votes rather than electorial votes? It isn't as clear as people think it is, because both the Bush and Gore campaigns would adjusted their strategies to earn the most popular votes. Still, a lot of Democrats felt cheated after the 2000 Election. A lot of Republicans would have felt cheated if Bush had won the popular vote and Gore won the electorial vote.