Is that there isnt a single issue of great importance that neither the right or left can, or are willing, to tackle. Or at least none that I can think of off the top of my head that could rally enough support for the rise of a single issue party.

Thats how the Republicans did it in 1860 by appealing to the large section of the country that wanted to do away with slavery. People often forget the party was originally an abolitionist party and that was where they gained support from, the people who thought slavery was wrong and deemed it necessary to get rid of it altogether.

The only way I can see the U.S. electing a third party candidate on a national level is with a single issue driving the platform.

What issue could possibly have that much influence today?