Probably for the same reason that Perry gets a pass for endorsing Guiliani. Duncan Hunter was out in mid January. Fred Thompson entered late and was out in January too. Cain's endorsement came in February. There wasn't exactly a stellar field left by that point in time.
Short of endorsing BO, there is no greater display of bad judgement than Cain’s endorsement of Romney and don’t be surprised if Cain endorses Romney again in 2012.
It is not hard to see why Cain endorsed Romney in 2008.
Romney owns the Largest Radio Network in the USA, Clear Channel Radio Network, and Romney is in a position to get Cain a lucrative Talk Radio Contract even if Romney does not win the Nomination.