I think the major issue with Hogwarts is the shape and massive size of the FJ show building. I'm guessing management realized that it would be difficult to cover the building from every angle without a massive undertaking and just couldn't justify the cost when it was only one piece (even if it is the centerpiece) of a land. I will admit they could have done a bit more, and I'm sure it would have added to the immersion, but they were probably running over budget and decided to keep cost low if they couldn't go all the way.
Volcano Bay is an entire park. FJ is one ride. Also, you have to remember that the Universal post HP and the expansion's success is far less timid than the one hoping to change their fortune with Hogsmeade.