Because they did have many options for how to display the roster (which everyone agrees was not made for the box). This is the logical way to do it:
Instead of putting them in batches of six, to fit the box, they put them in batches of nine, which means there are jumps in characters going out of numbered order (
on the box itself, not in the arbitrary pattern). Zelda comes after Sheik in numbered order, not Roy. Likewise, Palutena comes after Mii Fighters, not Corrin. It means, if I'm looking on the box, Roy is right after Sheik, which is out of order. I know the "pattern", but it doesn't make sense to put it that way on the box itself, this is. This tells me this is a placeholder image, not the final way the characters will be displayed. As such, it doesn't imply what characters are in, what characters aren't, the total number, etc. It's a placeholder. Subject to change.
I get that it's difficult to keep track of everything, as I'm having trouble myself. But I think my point is simple--if they're in ordered number on the box, it should be displayed as such, not inferred from some arbitrary "pattern" the designers of the box chose.
EDIT:
So you can see the subtle, but head-scratching, difference: