It may surprise you to know that in 2016, approximately 150 people registered with the Federal Election Commission as candidates for president of the United States.
We have criteria that are applied to anyone whose name has been registered with the FEC or who appears on any single state ballot in the U.S. Those criteria in the last few cycles have been three: constitutional eligibility to run and serve as the president of the United States; a chance to mathematically win the electoral college vote, because that's what matters here at the end of the day, as you know; and the candidate must meet 15% support in the polls approximately two weeks before the first debate.
That 15% is arrived at by taking five national polls that tend to be jointly conducted by newspapers and the large television networks. We average those polls—they need to be based on a very large sample survey—and see whether or not anyone, including the major party nominees, has met the 15%. If they have, they are included in the debate. The criteria are reapplied after each debate before the next debate, so that if someone's position changes, we can accommodate that.
Our debates take place in the last four weeks of the general election. You are right: there are a lot of other party candidates who would like very much to get air time. Increasingly in this country there are forums on different media outlets, particularly C-SPAN, that afford that opportunity and, of course, in particular, social media is a place where it is very cost-efficient to get your message out if you are a small candidate.
The short answer to your question is that the commission is chartered only to do general election debates between candidates who meet its FEC-mandated criteria. We cannot do anything that is especially targeted to accommodate candidates who do not meet that criteria.