[drivingmenuts]

The major parties (by failure to commit to a solid platform) are easier to agree with than the odd alternate candidate who may have a solid platform, but is only in agreement with a small percentage of voters.

More concretely, the Paul family might one or two things I agree with, but it's the other 999 things they believe in that I can't stand, thus I can't vote for them so much as against them.

[dragonwriter]

You don't really see vague umbrella parties without clear agendas as much in systems which have more effective electoral systems than the US's FPTP system. In an FPTP system, such parties are natural, since once you have established a position as a major party, you can win by the other side being disliked more than you, even if you aren't people's first choice (because the incentives in the electoral system are to vote for the least offensive of the major parties.)