Two words: Electoral college. As long presidential elections are conducted through that system, there will only be two viable parties that are competitive at the national level. Of course, becasue of that very same system people in safe states don't have to worry about voting for third parties, since voting for the other major party won't make a difference anyway.
Good point. The vast majority of people don't even need to worry about voting third party because they live in safe states. When the election isn't going to be close, nobody does.
Most people don't seem to understand this.