Rudy backed out of the race in 2000 because his campaign was unravelling before his very eyes. His tryst with Miss Judi and his disgustingly public announcement (press conference) notifying his wife of his decision to divorce caused him to sink in the polls (not to mention prosecution/conviction of many of his top aides).
Wimp Rudy couldn’t face his inevitable defeat and decided to retreat.
He keeps going down in the polls, though. People are catching on.