I’d say he could also wait until after. Last time, the vacancy motivated Republican more than Democratic turnout — and if Republicans don’t win the Senate again, there’s still time after the election to do the appointment.
There are almost a dozen GOP Senate seats up at risk. A vote before the election gives them a solid victory to campaign on to their base, which might be enough to keep those seats in the GOP in November.
Waiting until after the election does nothing to improve their electoral prospects, so for McConnell there's no point in waiting.
