From Theresa Amato writing for Vox:
Every four years, we come to this point: Dissatisfaction with the major party candidates cries for alternatives, and then reality comes crashing in. As an initial matter, the problem can be squarely laid at the feet of voters themselves as they are unwilling to take a risk outside the two parties once their nominees lose inside the two major parties.
I know because I tried twice to open up the system to candidacies outside the two parties: first, in 2000 as the national campaign manager for Ralph Nader when he was the Green Party nominee, and then again in 2004, when Nader ran on a potpourri of third-party and independent ballot lines.
Until we fix our Byzantine ballot access system, our partisan electoral administrations, our campaign financing system, our inexplicably exclusive Commission on Presidential Debates, and a media fixated on horse-race politics, it is a myth that anyone can run — successfully — for president outside of the two parties.