As I stated before, I'm a huge Star Wars fan and I don't have any interest in Potter. But even I can look at this objectively and say that Potter is currently the hotter franchise.
I agree.
It is a pretty straightforward supply and demand with regard to merchandising.
Right now, Star Wars fans are having their merchandising needs met across almost every platform.
Harry Potter merchandise, based on my own in store observations (anecdotial, I know), seems to be a fraction of what is available for Star Wars.
Intentional or not, Rowling has done a good job developing products in her universe that can translate satisfactorily to the real world. That chocolate frog may not move, but stick a wizard card in there and it is pretty darn close to the product in the movie. She has even set up a marketplace in her books that can translate to the real world.
Star Wars has got weapons that are physically impossible to make and nerf steak.
Even if Disney were to make a Star Wars land (which still wouldn't be a bad idea) marketing in-universe items that people must have would be difficult to near impossible. Beyond blue milk, what item in Star Wars are people scrambling for that is either: a. not currently available somewhere else, b. plausible to produce that would give the consumer an authentic "Star Wars" feel, or c. purchased in a place that evokes feelings directly from the movies?
In my opinion, that is why the Harry Potter land at IoA is so successful with merchandise. This stuff fits all of the above categories. You feel like you are buying a real Chocolate Frog from Honeyduke's. You feel like you are buying a real wand from Olivander's (granted wands don't really have to "do" anything, it's just a stick. Lightsabers need to have a stream of plasma shooting out the tip to be believable).
To sum up this rambling, in the theme park setting demand for Star Wars merchandise is low because of the proliferation of said merchandise in everyday life and the fantastical nature of the merchandise itself. Almost the exact converse is true for Harry Potter. "Real world" merchandise is more limited and most products for Potter have at least some anchor in the real world, making their production and conversion simplier.