Honestly, I would have to say it has to do with the audience and their most profitable lines. Star Wars is by and large their biggest money maker from everything I've seen. The UCS lines are catered to the hardcore fans obviously - TLG knows that people will buy those sets.
That being said, the modular line and larger sets like Tower Bridge and sculpture builds are also fair money makers for TLG. But those are more aimed at the AFOL vs younger children. While a 2500+ piece Hogwarts Castle would be an excellent model and build, would it offer the same allure as a large scale Castle (non-licensed). No, I don't think so.
I don't buy Star Wars Lego. I can't justify spending the money on those sets when I COULD spend it on sets I care more about (any building/structural set, for example). I had no problem buying the Tower Bridge for $200+, but a $130 Millenium Falcon? Nope. Themes have a very unique effect on people (and I'm no exception!). UCS sets go for insane prices in the after market, but I buy to build, so I'll use my cash on the sets I'm interested in.
Now of course, while I don't buy Harry Potter sets (I bought the new Knight Bus because of all the purple...), I am considering Diagon Alley because of the unique structures and piece types. Would I buy a big Harry Potter Castle? Meh...I don't know. It would have to offer more than just a Harry Potter memory for me to want to drop $150+ on it. I can only imagine that's how TLG looks at it.
Moving past that, Lego hasn't really announced a big non-Star Wars set since Tower Bridge. With that having been on the market for some time now, I do see them bringing forward a new AFOL inspired non-Star Wars super-build. It may be themed, it may not. I really can't imagine what it would be, but if history tells us anything, we won't be disappointed (Taj Mahal, Tower Bridge, Statue of Liberty, etc).