Several sound reasons for waiting:
1) If he delivers his report to the House Intelligence Committee before the 2018 midterms, then the committee can stall the release of the report, or not release it at all. This would force Democrats on the committee to either leak it or read it in full on the House floor, but the House is not in session from October 12 (last week) to November 13, a week after the election. This would also be a political risk for Democrats with the midterms a imminent. Like the Kavanaugh hearings, if there's a groundswell of momentum for Democrats, and very little motivation for REpublicans, you just don't know what will motivate turnout. It might have seemed unlikely a month+ ago, but the Kavanaugh hearings riled up the GOP base which had a malaise. It's risky that reading the report in a special session of congress might backfire in the midterms.
2) If you want until after the midterms, and if there is a Democratic house, and Democratic-led subcommittee, then the report gets release in full with no or few redactions.
3) Aside from those two major reasons, minor reasons are that Mueller is smart and knows the president is going to claim the report is trying to politicize the investigation, and this gives less fuel to that.
Most notably though, if you're a Democrat, this is probably a good thing that Mueller waits. Right now, Democrats have the advantage with voter mobilization. Like how the Kavanaugh hearing excited the Republican base, a damaging, bomb-throwing report could also motivate the Republican base ("This witchhunt is trying to impeach our president -- We have to come out and vote or they'll impeach Trump!")...
OR... Perhaps the report isn't as damning as what we already know, and then that could also help Trump.
In any event, it could be lose:lose releasing it before the mid-terms, and even if it is damaging to Trump, Congress isn't in session so Republicans could prevent it from being released anyway, the GOP base might turnout in higher numbers as a threat of impeachment, and then the whole thing gets buried if Democrats don't take the House. If Mueller waits, and Democrats take the house, then even if there isn't substantial bombs in the report, Democrats can launch investigations of different allegations throughout the report, Trumps financial history, and more.
Waiting is probably the prudent and politically beneficial thing here.
