Because the election laws that have been set up do not help third parties. Having runoffs between the top two in local elections is a help to the established parties.
The little parties need money and publicity. Perot had that in 1992 but failed to endorse or include people running for Congress, therefore the reform he had would have failed due to lack of support in Congress.
Essentially our laws were set up to protect the status quo. The Progressive party is the only one that lasted for a bit but they won too few elections to impact Congress.
