Another problem too is that the novelty of seeing animation and live-action blended together has long worn off. Roger Rabbit, while not the first example of such, was the most ground-breaking example of blending the two and it worked really well. Space Jam seemed to push the team-up of Michael Jordan and Bugs Bunny more than anything (considering they were doing commercials together prior to the movie). The Rocky and Bullwinkle movie kind of drew attention to itself in blending the two, and even referenced Roger Rabbit when Fearless Leader discusses destroying cartoon characters. Back in Action flopped for a lot of reasons, including apparently nobody liking Brendan Frazer anymore. In this day and age, where CGI rules animation, and makes it seem more believable and less magical that animated character and real people can co-exist in the same universe (Kangaroo Jack, Chipmunks, Smurfs, so on, so on), the magic just isn't there anymore.
Not only that, but as I pointed out, whereas Michael Jordan was the biggest name in basketball in the 90s, LeBron James has been the biggest name in basketball since the late 2000s, so it's already clear that whereas the original SJ movie was pretty much a vehicle for Jordan and Bugs, a sequel would clearly be nothing more than a vehicle for LeBron, who kind of has a little bit more celebrity status than Michael Jordan anyway . . . he was on SNL, and he was hilarious despite the fact that he can't act (but then again, Michael Jordan wasn't much of an actor either).