I'm not sure if the line dancing you are all talking about is what I have in mind, which is kind of like a group of people all dancing the same steps in tune to the music. If so, I have a theory that it is popular at least in part because it's a kind of bonding, which fits in with another theory I have that we have a built-in genetic inclination to want to work with others in small groups. Obviously some of us have less of that desire than others, but it's kind of a useful thing for evolution.
Maybe the same thing with music. I attended a history event at which a group of drummers and fifers was jamming with old Celtic music, not polite stuff but wild music that set my heart on fire. It was positively tribal. I think line dancing is a watered-down version of that. It seems like every generation has some form of it, too, from the Virginia Reel back in the 1800s to the Stroll in the 1950s to all the variations going on now.
Unless this isn't line dancing any more. In which case ignore everything I just said. :-)