Words are inadequate

Originally, the mystique. You could order papercraft pyramids, but if you wanted something with any heft or durability you had to make your own. And it was beloved by a number of people who at the time I approached with something a few notches shy of hero-worship. Later, when they first made the injection-molded plastic pyramids, the sheer prettiness. And also some of the really neat games that came out of it, that directly used the physicality of the pieces. Especially the original Icehouse (literally real-time strategy) and the induction-puzzle Zendo.