What debate did not have gotcha questions or biased moderators? And we complained. Candidates complained. But I don't remember any of them pouting about it and running away. Until now.
Not to shock anyone, but presidential campaigns employ strategic thinking. At the heart of that is the question, do debates make a difference?
Other candidates before have skipped debates. Some have even faired well despite their refusals to debate. Lyndon Johnson, Richard Nixon, Jimmy Carter all had refused debate.
Ronald Reagan skipped the last debate before the Iowa caucuses in 1980. He lost Iowa to George HW Bush, but went on to win the primary and a general election landslide victory.
Perhaps Trump has reasoned he can take a page from Reagan's history and make it work for him.