[WalterBright]

Who gets on the ballot is determined by laws designed to protect the two party system.

Even so, getting on the ballot requires campaigning, which requires money. For example, there is no ballot for 2020 yet, but there's a heluva lot of campaign money being spent right now.

[theandrewbailey]

There's also that nasty Electoral College thing that makes the presidential election effectively 50 sub-elections. That requires each candidate to register in each state to get on their ballot. No doubt, there's fees for that.