Gives Trump a chance to express the basis for his decision, and Fox/O'Reilly a chance to defend the network. It shows that Trump is open to a "fair hearing," let the listeners decide. Plus, there are issues that dwarf the conduct of one debate. Trump can play up that a debate without him on the stage gives the other candidates more time and room to sell themselves to the public. He can be magnanimous about it.
It’s all about power with O’Reilly. He want to come across as the know it all. I really don’t understand why he has to explain anything. It makes Trump look petty. If he’s going to be interviewed make it quick, but why let O’Reilly make you look bad.
OTOH...If O’Reilly agrees with him, then by all means do the interview.