Why american people only get two presidential candidates to chose from?

The two major parties have nationwide organizations to publicize their candidates and get the voters out to vote. Ad hoc third parties, no matter how popular among a segment of the electorate, can not come close to the vote pulling ability of the Dems and Reps. No "permanent" third party, such as the Socialist Party in the early 20th century had enough wide spread appeal to overcome the two big machines. The last 3rd party candidate to beat a major party candidate was ex-President Theodore Roosevelt running on the Bull Moose Party slate in 1912. He beat the Republican candidate, President William Taft, but lost to Democrat Woodrow Wilson. Since then, third party candidates have been popular, not popular enough to win an election but some of their platform planks have been adopted by the major parties. Socialists Eugene V. Debs and Norman Thomas would be considered main stream today. I voted for a sure loser 3rd party candidate last time. If enough people do that, maybe the big two will realize they're not reaching many voters with the mediocrities they put up.