Why do third parties usually not last very long in the american two party system?

Atreaty was signed in 1842 to end the border dispute between the u.s. and britain in maine. which best expresses the end result for britain? *pic* the british lost all the territory in maine the british territory was bigger than that of the u.s. the british territory was smaller than that of the u.s. the british territory was the same size as the u.s. territory