You have to understand that Movie World only has a connection to Warner Bros. because they licence the name for use. Warner Bros. don't have any control or share in the park at all, they don't own anything. So every single IP usage you see is under licencing agreement. Now there are all kinds of agreements that licence their use for all kinds of things. Just because you have the image rights to use a particular character for live action characters or statues, doesn't automatically mean you can build an attraction or sell their likeness of merchandising. It's all negotiated upon. The more you want to use, the bigger the $$$$$$
You are forgetting about the yogi bear 3d movie and recent Tom and Jerry usage too. Plus the use of characters in the park and in merchandising. It's not like they haven't used any at all.
It's pretty simple really. You want to pay for things that are current and relevant in the public domain in order to get your best return for the investment. There is a lot of HB stock that really isn't known to the current generation (even the previous generation) of kids. So you have to be careful about where you put your money.