Release dates & Ike. Most reveals were Fridays. However, several were clearly not. So Friday is not the only day they can be revealed.
The short answer is that not all of the rest will be revealed until post release, being hidden.
The alternate is that many of them will be revealed in Newcomer videos, as Sakurai implied they have a few left (and they HAVE to come out BEFORE the game, because the videos are available from the start in the game itself).
There's 11 characters unaccounted for, 14 if you actually think Snake/Squirtle/Ivysaur are back. They could easily just reveal them all in a Direct like Mario Kart 8 did, or split them across videos, do random reveals, obscure release reveals, or have some saved for the final game.
The only ones that 'have' to be revealed are Wario, Ice Climbers, Ness or Lucas & Meta Knight, and there's plenty of time for it.