I think Disney is all about marketing and anticipation. They know the opening of a new attraction or land will bring more guests to the park to see it. Why not take their time and aim to open things during peak season?
Look at how Cars Land went. DCA was a maze for a long time and they spread things out to open the "new" DCA in time for summer. I'm sure part of it is making sure things are done correctly and giving enough time to add all the little details, but I think it's more of a marketing tool. Not to mention it gives them enough time to get in compliance with various laws of each state.
Cars Land was the same thing. Spreading the costs out over multiple years, in this case it was the entire park redo project as that was in the unusual place of being one budget for everything included. It just had a far bigger payoff than a nice, little, D-ticket family coaster. Fantasyland has fallen flat in its marketability because there is so little of substance.