What were they thinking?
Accessibility (for maintenance now and forever)
Visibility/Full understanding of guests about what's inside the volcano
Natural lighting (something I think is important at a water park)
Openness while queuing for stuff inside the volcano
Cost - but not construction. An enclosed volcano, in Florida, with water, would become a miserable sweatbox that would require massive, industrial air handler units. Those would make the inside cold and the outside blistering which, again, is bad news for a water park.
I understand being a bit confused by the park layout, but it comes across as if a lack of understanding created expectations. Expectations have to shift when knowledge is gained.