The correct answer is the following.
The Electoral College discourages third parties because they consider that a third-party candidate must have a full and proven background with a solid and ample political base that can support him/her. Usually, third-party candidates do not have the money to establish a formal and large team to run a political campaign. Often, they not have the resources to compete against Republican of Democratic candidates.
Among the recent independent candidates that have more presence in the campaigns was George Wallace in 1948, Ross Perot in 1996, and Ralph Nader in 2000.