The debates are not publicly-run events

There's a minimum requirement to participate

It’s pretty rare for a third-party candidate to make the debate stage

The first presidential debate is finally upon us.On Monday, our candidates for commander-in-chief will finally go head-to-head on issues that matter to Americans. But not all the White House hopefuls will be part of the rhetorical rumble.The presidential debates are traditionally limited to the nominees from the two major parties; in the 2016 election cycle, that means Republican Donald Trump and Democrat Hillary Clinton respectively. But with dissatisfaction with the establishment candidate on both sides of the aisle, some voters are considering third party candidates like the Green Party’s Jill Stein and Libertarian Gary Johnson. As The Washington Post notes, Johnson is polling the higher than any third-party candidate in the last 20 years.So why won’t we see them on the debate stage on Monday night? Here are a few reasons:Despite the public-service nature of the debates, they’re not actually government-run. The debates are organized and run by the Commission on Presidential Debates (CPD) , a private organization that receives no government funding. The CPD’s board of directors includes a number of former Democratic and Republican politicians, but is not affiliated with either party.While the CPD’s legal status as a 501(c)(3) nonprofit makes it eligible to host and sponsor debates, it’s under no obligation to open debates to anyone it doesn’t want to. It is required to remain nonpartisan in order to keep its tax-exempt status, meaning that it can’t endorse one candidate or party over another.No one is automatically invited to the debate, no matter what party they come from, according to the CPD. The organization requires that a candidate win support from a minimum of 15% of voters in at least five national polls to qualify, as well as be on the ballot in enough states that they have a mathematical chance of winning the electoral college.The two biggest third party candidates have both met one requirement, but not the other. Libertarian Johnson will be on the ballot in all 50 states, while you can vote for Green Party's Stein in 45 states and add her as an official write-in candidate in three more. But Johnson only pulled in about 8% of the electorate in the polls used by the CPD, according to NPR , while Stein took about 3% —both respectable numbers for third parties, but well short of the threshold.“It was the CPD’s judgment that the 15% threshold best balanced the goal of being sufficiently inclusive to invite those candidates considered to be among the leading candidates,” the website states, “without being so inclusive that invitations would be extended to candidates with only very modest levels of public support, thereby jeopardizing the voter education purposes of the debates.”In fact, it’s only happened once before. In 1992, independent candidate H. Ross Perot participated in the presidential debates, and did well enough that a number of polls after the first debate declared him the marked winner. Though he didn’t win any electoral votes, Perot ended his campaign with the best-ever result for a third party candidate, taking home almost 19% of the popular vote