Because Donald Trump is threatening to run for President AGAINST the Republican candidates. Why would GOP candidates want to attend a debate moderated by someone who is actively discussing running a 3rd party indie bid against them. It's just silly. Further, Trump is a realityTV show host, and the setup of him hosting a competition, judging the performance of the candidates and picking a winner (his endorsement) afterwards is strikingly similar to his realityTV show format. The entire thing is silly and I am glad enough of the candidates have rejected it that the thing will hopefully just be canceled.
Bloomberg is on that list. He has talked about running as a third party candidate. Did you object to that debate or just don’t like Trump? Most of those news outlets are the avowed enemies of everything we stand for. Why does the Republican party allow anyone to the left of Fox News (who really aren’t that conservative) to host our debates?