There are structural reasons why this is so. For one thing, if a third party's position on an issue becomes popular, then one or both of the major parties will co-opt that position. The Democrats and Republicans both began co-opting the various socialist parties' economic policies after 1933.
If third parties wish to influence public policy, they must focus on changing the culture. On changing attitudes and opinions. This means fearlessly promoting their principles (i.e., the dreaded libertarian macho flash). Once such uncompromising promotion influences enough voters, one or both of the major parties will implement that third party principle into a concrete policy position.
In the period 1872-1920, all political observers agreed that virtually all people who voted for the Prohibition Party would have voted Republican if the Prohibition Party had not been running.
Prohibition Party cost the Republicans two presidential elections, 1884 and 1916. After the 1884 election the Prohibition candidate, John P. St. John, was hung in effigy or burned in effigy all across the north, but angry Republicans.
But after the 1916, when the Republicans again lost the election due to "spoiling" by the Prohibition Party, the Republicans in Congress decided to pass the prohibition amendment, which had been pending in Congress since 1875 but which had made no headway. The Republicans were sick of losing over the prohibition issue.
This shows the power of minor party voters. "Spoiling" is a stupid name but it is a potent weapon and voters should not be intimidated into being afraid to use it. It is one of the few ways a small minority can have some leverage.
Also never forget, in 1776 only a minority of Americans supported independence from Britain. You don't need a majority to change history.