Surprised his opponent or his party didn't do some vetting. the gop only has a very small majority. If this came out before, it could have cost them. Seems like he lied about everything on his background.
Had they not, the Democrats may have retained control.
I can only guess, but it's ambiguous, irrelevant and needlessly casts this as a partisan issue.
Implicit in your statement is that oppo research is at least as good as the journalism machinery. Perhaps the answer to your question is that they did some vetting, but are nowhere near as effective as the press. It's fashionable to dunk on the press (or "mainstream media") lately, but perhaps they have a useful function whose effectiveness does not wax and wain with the election seasons.