It has been bothering me how long Mueller is taking, but there is a chance that he needs to wait till the democrats regain control of the house and senate. There is not much point releasing findings if the GOP will not impeach trump. Likewise, if the DNC only gets the house and not the senate, the best we can really hope for is impeachment by the house, but no conviction by the senate. In other words, trump would be removed, but that would be the extent of his punishment.
Edit: Sorry, had it in my head that the house impeaches/removes and the senate does convictions. The correct way, as discussed below, is that the house impeaches and the senate removes/convicts. So an impeachement with a GOP controlled senate would do nothing.