"Why was it so hard for Obomba to put that to rest?"
______

Because there are enough stupid people who believe if it's repeated, re-tweeted, cross posted etc. etc. long enough "it must be true". 
That's nothing new, has been like that for a very, very long time.  In recent times one in the trinity on top of that manure heap was Hitler. 
Is that trinity (H, S and ...) to be dethroned?