Because realistically, I still don't see Nintendo characters dressing up for Halloween especially for daytime operations. If Disney reserves most of their Halloween stuff/costumes for their own nighttime Halloween events, I doubt Universal would put out the same for SNW characters for normal/daytime operations before Halloween night. I just don't see that Universal would do something like that in real life.
As for whether SNW could be open as a scarezone or not for HHN, my reasoning was since Booville, a non-scariest place for HHN 2002 yet classified as a scarezone, could open, then SNW can do the same as well. I guess that Booville in 2002 was very different than the current SNW, but again, it's not that far fetched. SNW doesn't have to be scary, but it makes much more fun if it has Halloween overlays and Nintendo ghoul-like characters roaming around the area. Nintendo also makes music that are creepy and doesn't really shy away from it at all. Play some scary Nintendo music especially if most of the attendees are mature people along the way similar to The Simpsons' Treehouse of Horror opening/ending credits at the Upper Lot. Again earlier I said, I'm not saying it should be that scary, but it would be a nice to have some Halloween overlays and some scary music rather than being generic.