Not because of any ideological reasoning, but because of pure practicality and common sense.
Obama is afraid of Romney, not Cain. Even if he did go to the absurd lengths of taking such a colossal risk, it would never be against a clown and a buffoon like Cain.

Besides, no one in his right mind would take a chance of being discovered framing an opponent until AFTER he became an actual opponent--that is, after he was actually nominated.

In fact, Cain was no threat to anyone except Gingrich, (and earlier, Perry),and then only in the context of the GOP nomination race. You will recall Cain blamed Perry for the original accusations.

For the record, I believe Cain is probably innocent of these latest charges, and probably guilty of those National Restaurant Association gropes. There was actual smoke back then, and that happened way before Cain announced he wa running.

In my opinion, this was a Gingrich wet job that was accidentally delayed a few days.