I think elements of the mainstream press were who leaked it to Drudge, perhaps through channels (ie Lehane). THEY have a story (which is what reporters care about more than politics), but their editors won't let them report it without proof; they need things to be fairly out in the open, have some justification beyond unsubstantiated allegations.
As has been pointed out, there's no court case, no charges, no nuthin', unlike the Lewinsky scandal. All they have to work with at this point are unsupported allegations and non-denial denials, but now that the water is well and truly bloodied I think all will come out eventually.