They were easy to make with uncomplicated circuitry. That's it. The original big modulars included oscillator sync - which locked a bank of oscillators togethe

They were easy to make with uncomplicated circuitry. That's it. The original big modulars included oscillator sync - which locked a bank of oscillators together - and waveform mixing. So you could create more complex shapes. (With some effort and imagination, and not in a particularly intuitive way.) When digital started to become a thing in the 80s you could start using any shape, including your own hand-drawn waveshapes or a custom mix of overtones. But that turned out to be less exciting than expected, so most people just switched to samples instead. A sawtooth is not heard an octave lower. The fundamental is the same for both shapes, because the Fourier decomposition has the same f0. There are some unexpected psychoacoustic effects which change the perception of fundamental pitch. E.g there's almost no fundamental in the lowest octave of a piano, but our ears fill it in anyway. But simple repeating shapes tend to do what it says on the label. (And in the textbook.)