The Zamperla rides were all leased, and were not ordered in time to arrive on park for the start of the season.
My working presumption is that they knew Nightmare Niagara probably wouldn't run again but they believed The Missile and the Rapids would. Over the course of the closed season it became clear that both required significant investment, which wasn't forthcoming. I think the 'family focus' plan was always the strategy (there had been a couple of cheap kids rides added in 2004, I think), but their hand was effectively forced by the compound effect of years of neglect under THG.
Attendence, predictably, fell off a cliff and didn't recover significantly in 2006 either.