Again, this isn't simply a less popular IP. It's one that almost killed Disney.
Song of the South is an odd choice for... other reasons, but it was a success and produced a hit song. Besides, it belongs to the larger "Disney Animation" IP.
At the moment, Marvel has almost zero representation in the parks. And both Marvel and Star Wars are among the top IPs in the world, with only Potter contending with them - and not that closely. Each could support a full park (we all know the problems with Marvel in Orlando, but they do have access to some of the properties.)
And then there's Indy, which I'd really love to see better represented.
What I'd LIKE to see is a bunch of originals with a few IPs sprinkled in, but that ain't happening.
As to variety, that's not a great point in Tron's favor. It's going next to a very similar ride while another similar ride is being added to EPCOT.
PS: Eh, you know what. This was never a major complaint, just a goofy nitpick that I found amusing. I don't have a problem with the use of the IP, its just funny in context. My real complaints about the ride pertain to its redundancy.