Kush balls were just little rubber balls with rubbery spines all around them. Why were they fun? Why is anything fun? Why do people love fidget spinners? Who knows. I'd argue that any form of a ball is inherently fun. You can always find something to do with a ball, especially one that can be shot like a slingshot. For real. You could grab one of the spines, pull back, and then the ball would go flying. That's way better than a normal ball.
Comedian Rosie O'donnell made the Kush balls famous by throwing them during her show. I don't know how that started, and it feels like a waste of energy for me to research it, so I won't. All you need to know is that she did it and all of a sudden stores were selling out of them. Now that's power.