I'd say they won't say a date yet partly because they of course want to be careful about construction delays, and partly because they don't want to completely kill second quarter stays and park attendance. As long as there is no firm date, people seem to be okay to book in June and "hope". But if they announced a date, a lot of those folks might postpone.
On the bright side, late May and early June might be the best times ever to see Hogsmeade without crowds... as many people will "wait" for the other half to open if they can.