I know you were probably referring to the presidential candidates, but at the local level, there are Green Party mayors and councilors. There is also an independent (though social democrat leaning) congressman running for president now, though he is running as a democrat. The US electoral system is screwy enough without the electoral college, but with it, it makes it much harder for a president to be from a third party. Personally, I think the president should be like a city manager at the national level - apolitical and unbiased, without veto powers, but that is just my opinion. Without a change to ranked choice voting (proportional representation would be hard as representative per person is already abysmal) and abolition of the electoral college, likely not.
Both current parties have in place a highly organized infrastructure to bring up candidates up through the system. Those who can raise the most money for their respective parties is as important as running for office. Everything is linked up with corporate interests and lobby groups. An independent cannot develop this kind of clout. It's unfortunate but a fact of our current system.