One reason Avatar has taken a while from announcement to completion is that they had to move the Lion King show before construction on Pandora could start.
Similarly, they have to move something's at
Disneyland before they can start work on Star Wars there, and they probably want to open it at DHS simultaneously with DL or close to it.
It makes a lot of sense to phase these openings so something new is coming every year or so. Too much sooner and you lose repeat visits, so you are spending money faster and not making as much during construction - people will delay a trip to see everything, or they won't come the next year because there is nothing new.