Well.....
They didn't want to shut down DCA to do the renovations. ; If they did it would have been done by now.
The Board approved the World of Color show before the 1.5B. ; So right now the lagoon is closed for renovations, along with the lakeside area to create a viewing area. ; The site for The Little Mermaid is right next to the lakeside, so closing it before the viewing area is open would create massive traffic jams.
Then there is the fact that they had to tear down the show building that it is replacing (Golden Dreams film narrated by Whoopi Goldberg, who was also a major star on Superstar Limo). ; To make matters worse, the only bathrooms in the vicinity were in the same building!
So they had to build the bathrooms first across the street/walkway before tearing down the old building.
The bathrooms just got finished this month, along with the first wave of the Paradise Pier redo after Toy Story Mania defined the new Pier area. ; So as soon as the viewing area is done, they can demo and begin construction on the show building.
I believe they've already closed what's left of the old parking lot and started ground prep for Carsland and Radiator Springs Racing. ; DCA took 3/4 years to build from the parking lot up, so I know it will take at least 3 years to do the same for Carsland.