The bigger reason is Modbus is seen as being complicated. Which for somebody non-techy, it is. Get a single setting wrong, and it won't work, or at least not as expected, with no easy way to test why for your average home CNC'r.

Combine that with VFDs and Modbus equipped VFDs traditionally being expensive, meaning basic 0-10V DC speed controllers were far more commonplace, and you can see why 0-10V is the more commonly used method for home CNC. Plus it's easier to diagnose, as you only need a multimeter to narrow down where the problem lies.