Because he can't.
http://www.sfgate.com/cgi-bin/article.cgi?file=/chronicle/archive/2004/09/15/MNGHO8P5531.DTL
These are the three presidential debates and one vice presidential debate proposed by the Commission on Presidential Debates. All are scheduled to begin at 6 p.m. and last 90 minutes:
---------------------------------------------
Presidential
THURSDAY, SEPT. 30
-- Where: University of Miami, Coral Gables, Fla.
-- Moderator: Jim Lehrer, anchor, "The NewsHour" on PBS
FRIDAY, OCT. 8
-- Where: Washington University, St. Louis, Mo.
-- Moderator: Gwen Ifill, anchor-reporter, "The NewsHour," and moderator, "Washington Week in Review" on PBS
WEDNESDAY, OCT. 13
-- Where: Arizona State University, Tempe, Ariz.
-- Moderator: Bob Schieffer, CBS News, moderator of "Face the Nation"
---------------------------------------------
Vice Presidential
-- When: Oct. 5
-- Where: Case Western Reserve University, Cleveland, Ohio
-- Moderator: Charles Gibson, ABC News' "Good Morning America"
Source: Commission on Presidential Debates