Indictment wasn’t the goal. Impeachment is.
Mueller wants to get enough damaging info. on Trump and pass it on to, what he hopes, is a House run by the Dims, who will vote for impeachment.
Of course, the Senate is going to remain under the control of the GOP so no chance Trump will be convicted.
It takes a two thirds vote in the Senate to convict. But a Dem run impeachment would gum up the House for months.