We don't because the electoral system is set up to perpetuate a 2-party system.

Let me explain:
When voting for senators and representatives, we use plurality voting. This means that the candidate with the most votes is elected, and people can only vote for one candidate. A vote for one candidate is identical to voting against the others. So people vote against what they fear, rather than for what they want.

According to Kenneth J. Arrow, there is no truly fair system of voting (the Impossibility Theorem). The most fair system currently known is called "Approval Voting". It's just like plurality voting, except that everyone may vote for as many candidates as s/he likes. One person may only vote for any given candidate once (or not at all). This has an advantage over plurality voting in that it is no longer necessary to vote only for the lesser evil; you can vote for Al Gore and Ralph Nader, if you are willling to have either of them as president, but not George W. Bush or Pat Buchannan.

The day this is implemented in the US is the day pigs grow wings and start flying out of my ass.