Here's a reason they won't extend the lagoon:
Capacity. I believe it's been stated that Cabana Bay will have more rooms than any other Universal Onsite Hotel (please correct me if I'm wrong). Because of this and the pricing, more guests will be staying at Cabana Bay than any of the other on site hotels. Because Cabana Bay will be the farthest away, and because there will be more guests traveling from Cabana Bay to the parks, then it will take a lot of time to transport guests, therefore testing their patience. It also means that Universal will have to spend more for fuel, and possibly buy another boat and hire more drivers. Or, they could replace the boats with a fleet that has a higher capacity.