As far as I understand, I'd have to follow up the research... but both the R and D National Committees are required to hold debates if poll support for a candidate surpasses a certain threshold.
So a candidate meeting that threshold can demand the debate, or acquiesce, to the incumbent.
For example: https://www.politico.com/story/2019/...debate-1508516
"Candidates will need to clear 3 percent in four DNC-approved polls, up from the 2 percent required to qualify for the September and October debates."
This time the DNC is just deciding, nope! not going to do it at all... even though there are 2 other Democrat Presidential candidates polling over 5%.
So fuck the voters. Fuck their own regulations. Fuck the process. You will only vote for who they want you to vote for.