Getting an external company to build and operate a Hotel, only to later have the park take it over seems to be a common industry trend. Off the top of my head both Disney and Chessington have done this, I'm sure there are many more examples.
Normally it happens because:
1 - The park wants a Hotel but can't fund it (The original Disneyland Hotel)
2 - The park doesn't have experience running a hotel and for a small hotel it's not profitable for them to run it themselves.
3 - Having a well known external chain can encourage people to stay who wouldn't have otherwise.
4 - local authority's require competition to prevent a monopoly forming.
I have no idea what the reason is in this case. Potentially it's number 1; they had the budget for a Waterpark or 3rd Hotel but wanted both.