Short Answer: Because there's no there, there.
In April, Mueller referred an investigation of close Trump associate Michael Cohen to federal prosecutors in New York. This month, the U.S. attorney in Washington — not Mueller — indicted Maria Butina on charges of being an unregistered Russian agent. And also this month, when Mueller charged 12 Russian intelligence officers with hacking Democratic Party and Clinton campaign offices, he immediately turned the case over to the Justice Department's National Security Division for prosecution.
Mueller started transferring cases when he decided to grandstand on the first bunch of Russian cybertrolls and a Russian company that did not exist (Concord Catering) at the time of the alleged collusion. Mueller is thought to have done these indictments as a publicity stunt but he got his rear kicked by the Russian firm showing up in a US Court with their lawyers demanding the evidence used by Mueller against them. That wasn't supposed to happen.
So Mueller had three choices:
1. Turn over the evidence and suffer humiliation for how flimsy it was (Mueller said it would harm national security).
2. Withdraw the case, which would be a huge embarrassment to Mueller and his investigation and would most certainly be followed by calls from Congress to shut his office down.
3. Transfer the cases to other DOJ attorneys, run away, and escape blame.
Mueller chose No. 3 and is doing the same with other flimsy cases. His purpose has been to slow down Trump from indicting Rosenstein and others in the illegal spying of the President and to monitor what the President's team is doing in regards to the Hillary and DNC emails and servers as well as other things.
“Mueller chose No. 3 and is doing the same with other flimsy cases. His purpose has been to slow down Trump from indicting Rosenstein and others in the illegal spying of the President and to monitor what the President’s team is doing in regards to the Hillary and DNC emails and servers as well as other things.”
My take is that this whole charade has had as its goal to tie up Trump to the point that he can’t get things done (reversing the Obamanation of Desolation). That hasn’t worked out too well for Mueller and the rest of his #Resistance goons either.