Why are there usually only Republican and Democratic presidential candidates?

Why are there usually only Republican and Democratic presidential candidates?

Thea Brodkin (Voter Service Chair) gives expert video advice on: How are presidential candidates selected?; Can I affect who is selected as a presidential candidate?; What happens at a national convention? and more...

The way our system runs is the two major parties sort of have control over a lot of it, so it's very hard for a 3rd party to get started and raise the resources and the money and the backing that it needs. You have to get on the ballot in all 50 states to really become a national candidate and it takes a lot of money. So it's very hard for a 3rd party candidate, whereas the Democrats and Republicans do fundraising all year for 4 years before the election, so they're ready to go.