In some states the major parties got certain candidates banned. Here in indiana the Green party is banned even as a write-in (try to make that make sense) but Cornel West as an independent is accepted and so is PSL candidate Claudia De la Cruz. In Georgia Cornel West and Claudia De la Cruz are banned while the Green Party's Jill Stein is allowed. It's worth checking out each candidates website to see which states they have "ballot access"...

[addendum] I feel like i'm committing an injustice if I don't mention the Libertarian party which also has ballot access in every state, though I adamantly disagree with them.