Because polling currently suggests that there's a bug leftward shift in American politics coming. If the special election were to happen now, there's a non-zero chance that the GOP would lose the seat to a Dem.
Hiding McConnell's death would mean that, even though he wouldn't be voting, the Dems wouldn't get an additional vote in the Senate early.
That's the justification, but it's still just a conspiracy theory. Also, I'm not a poli-sci person, so I don't know about Kentucky's special election rules or if Mitch's seat is being contested this year.