Actually, my understanding is that when they sold those packages they were actually expecting it to be open, not in softs. Softs are typically not announced to the public, just one day it is open and the next day it could be closed. This would happen all the way until the actual grand opening.
I see. However, they definitely sold those packages way in advance and someone must have made a decision at some point at time that they would at the very least have soft openings going on for those guests if they weren't ready to officially open yet, which is exactly what happened. I was one of the people who purchased a WWOHP vacation package, and I went from being shocked and confused when they announced a grand opening date
after my vacation dates to relieved and surprisingly pleased when I discovered that although WWOHP wouldn't be open all day, it would be open in the mornings to vacation package guests only. A few hours in a minimally crowded DA beats opening-day DA crowds hands down.
I would think they could fill their on-site hotels even before the grand opening if they began selling WWOHP-Diagon Alley packages that guaranteed guests would be able to visit DA at least a few hours each morning whether it was officially open or not. The only reason I can imagine they wouldn't start selling DA vacation packages is if there is some important aspect of DA (e.g. Gringotts, Hogwarts Express, or the new Ollivanders) that is so completely dysfunctional that they are completely unable at this point to predict even when it will be ready for partial-day soft openings.
Edit:
Either way, I would think a good way to avoid the opening day headaches they had for Hogsmeade's opening would be to announce a whole official opening week, with celebratory events that get better and more exciting as the week progresses. Monday: DA is open! Owl-balloons released at 8am and fireworks at closing! Tuesday: 100 Bronze Knut tickets for free Carkitt Market butterbeer distributed at random to guests in both parks, fireworks at closing. Wednesday: 100 Silver Sickle tickets for free icecream at Florean Fortescue's, fireworks at closing. Thursday: 100 Gold Galleon tickets for a free entree at the Leaky Cauldron, fireworks at closing. Friday: 30 deathly hallows tickets for a free interactive wand, appearances by Robbie Coltrane, Evanna Lynch, and James and Oliver Phelps at various locations throughout the parks, and yes, fireworks at closing. Or something like that.