They have been brainwashed by the big two into believing that a 3rd party candidate is a vote for "the other person".
In most cases, it is. That's a fact. Hence the "lesser of two evils" vote pattern.
Now, if a party were to arise that actually approached the constitutional model of government in a more authentic and effective manner than the two dominant parties, we'd be talking.
There's also that little fact that education is the key to good government, because it is filled with would-be P.T. Barnums looking for that next sucker. Compared to even fifty years ago, American citizens are civic morons. That was accomplished with intent.