Something happened behind the scenes. I think the "respect the families wishes" was the line they gave him to back out of the story. Something much larger at play. I don't fault him but think you are correct he will not be leading the charge on it.
It is my speculation that the Trump Team gave Sean information that revelations about Seth Rich were in the works. The effort should be to cut off the head of the snake. To give a mortal blow to Russiagate would be the revelation that Seth Rich was the source of the DNC email leaks and a person was arrested as his killer. I think President Trump already knows and once he has his F.B.I Director in place and the intelligence community cleaned out of the Obama holdovers, we should see the dominoes fall.