Cain endorsed Romney once it was clear the Romney was the only choice apart from McCain (and maybe Huck).
I ask the same question I asked about Rush Limbaugh:
If you are a conservative, hoping the GOP will nominate a conservative, given the impact your endorsement may have, why would you wait to endorse anyone until your endorsement has turned into nothing more than an ineffectual cry for help?
Romney wasn’t any real plus over McCain. He was just a product of the “anyone BUT McCain” reaction. Too late for any real impact on the process. Cain is simply not a reflexive conservative.