I think the issue at Animal Kingdom is that it would be a long walk past a lot of empty land. At EPCOT you're walking through Future World (I know that's not the name anymore but it's an easier way to describe the whole area) and then past World Showcase countries. At AK you'd have about a half mile walk through nothing to get all the way up to where Rafiki's is now.
Of course they could develop the land north of the Jungle Trek/Kali and east of the Safari, but I'm not sure what kind of restrictions they'd have in that area due to its location near animal facilities.