Mueller only just indicted the Russians for interfering with the election 20 days ago. You can’t charge Americans for the crime if there was no crime. Having confirmed that there was a crime committed by Russia, it can then follow that Americans could be involved.
Further, Manafort’s crimes are specific to him, and so you can move ahead with charging him for those crimes. General coordination between the Russian government and Trump’s campaign team is a larger act of conspiracy between a wide body of people.
If you try to prosecute those people one by one as evidence of their misdeeds come to light, then you have to release your evidence. That evidence will likely implicate others in the crime as well - others who have not yet been charged. This allows everyone who has not yet been charged with anything to work out a shared story that allows for the evidence to exist while not implicating themselves. It’s when people tell different stories that you can play them off against each other to force them to start telling the truth. If everyone’s telling the same story, then you have to accept that as the truth unless you can find documentary evidence to the contrary.
In end result, if you want to get everyone who was involved, you can’t prosecute the crime against any one of them until you have determined who all was involved, to what extent, and have all of the evidence that you will need against all of them.
There are certain individuals, like Manafort, whom we have no real doubt were working with Russia to coordinate something. That he hasn’t been charged with that seems like a fair indication that it’s still coming. And if Mueller has looked through everything Manafort has done so thoroughly as he obviously has, AND gotten his close confidant to turn on him, then it’s reasonable to say that either there is nothing at all, or that there are more people in on it than just Manafort and the case is still being built against them.
As I noted in my first post, there is nothing in Mueller’s mandate that would lead to him making indictments against Russians. He was specifically directed to investigate Americans in Trump’s campaign organization. So if he is charging the Russians with those crimes, then clearly that came as a direct extension of investigating those Americans, and as preparation for laying the basis of the crime that the Americans cooperated with.
So if the question is that there is nothing at all or that he’s still building a case, the safe money is on building a case.