While I have no specifics to back this up (AKA Stop reading this post now) I have to wonder how long they were working on it. Its not like they went down to Film-Mart and saw a sale, so they bought it, and hoped the Wife wouldn't notice it when they got home. These things take time, and negotiation, analysis, and maneuvering. It certainly wasn't a quick decision. If it was even a consideration at the time when Harry Potter rights were up for purchase, I don't know.
Having said that... Lucasfilm was a huge buy in, not just theme park rights. They own the entire thing. It is difficult to compare them in any way. Now Avatar and Harry Potter are at least similar in scope, if not scale.