15% Polling Requirement:
The Exclusion of 3rd Parties
In 2000, the Commission on Presidential Debates (CPD) established a rule that for a candidate to be included in the national debates he or she must achieve at least 15% support across five national polls.
The five national polls all have excluded both the Libertarian Party and Green Party candidates' since June. However, they chose to include the Democratic Party's assumptive candidate months before his official nomination in August 2020.
Is it typical to only poll on two candidates?
Nope. When looking at other polls such as the Republican & Democrat Primaries, those same polling organizations included large lists of candidates and even asked the respondent who their second choice would be or if they would be voting for someone not named in their list.
Exclusion in Polling
See below images of how inclusive the Washington Post - ABC Poll is towards Democratic Presidential Candidates in the primaries. Yet when it comes to polling for the Presidential Polls that they know will be used to determine the debate invitations, they only offer two and do not allow for other options.
The Washington Post - ABC News Polling Example: