I really don't think it has anything to do with Harry Potter being more popular or more familiar. People find the theming at Hogsmeade and especially Diagon Alley to be more detailed and elaborate than the theming at Pandora because the source material is vastly more detailed and elaborate. Harry Potter is the perfect IP for a theme park land because Rowling and the films created a fully realized world with iconic places and things like Hogwarts Castle, Diagon Alley, Gringotts, the Hogwarts Express, the Knight Bus, Butterbeer and extensive detail down to the various intriguing candies sold at Honeydukes, the flavor of ice cream eaten at Hogwarts feasts, and the appearance of the toilets used to enter the Ministry of Magic. Avatar is a MUCH more simplistic story than Harry Potter with much less material to work with. Having said that, it fits Animal Kingdom reasonably well and Disney did a very good job with what they had to work with.
I also do not see how Pandora can be considered more "immersive" than Diagon Alley. Pandora provides a very impressive faux exotic rainforest setting but at no time do you actually feel that you are in a rainforest or on an alien planet. When you walk through the entrance to Diagon Alley (with no outside attractions visible) and look in front of you, you feel that you have just walked into the fictional setting described in the books, not a theme park imitation of that setting. I agree that the roller coasters are a problem at Hogsmeade, but if there are any roller coasters visible from Diagon Alley, they aren't easy to see.