Besides the fact that in a lot of places there are laws against going on the ballot if you aren't one of the two major parties (talk about undemocratic!), one of the major problems is the 'first past the post' system of American elections. No one votes third party, because they're afraid of wasting their vote, and hurting the side they don't hate most.
You know that if we had a ballot system where you could, for example, rank candidates with first place getting three votes, second getting two, third getting one (a proposal I've seen), the major parties would be doomed in short order.