I believe there a couple of reasons, here are a few:
1) As stated by a PP, Disney is very passionate and mindful about "preserving the magic" for current guests, this means they avoid major construction work during park operating hours (that guests could hear) and have a limited space to work in behind tall construction walls to avoid the sight of construction ruining the "magic" for current guests..
2) Disney is a GIGANTIC company with many layers of management and red tape, something as simple as changing the color of a wall in a themed area of a park requires layers of approvals from different divisions and executives. All this red tape tends to slow down major projects at Disney. Once again this goes back to Disney being very particular about "preserving the magic" and ensuring that any major decision that is made is well thought out from both a strategic business perspective as well as creative perspective ensuring that it fits in with the themes and "magic" of Disney Parks.
3) Disney is already the most successful theme park operator in the world and has proved itself as the biggest player in the industry on a global scale, this means that there is no sense of urgency from WDW executives to draw more market share when people are already coming in droves into existing WDW parks and resorts. New attractions, lands, and resorts are added bonuses to bring even MORE people, but it is far from a critical necessity.. Other competitor parks such as Universal have a greater sense of urgency as a project such as WWOHP WILL bring in NEEDED increased attendance numbers for those parks and make a MAJOR difference on their bottom line.
5) In regards to AVATARland, I believe there is also a strategic business-driven reason behind the delay involving the release of the new sequel movie. As stated by PP, timing the opening of AVATARland with the new movie would be a smart course of action to use buzz from the new movie as fuel for the new land.