This is two-fold. Firstly the cost and availability – you can go out and buy a replacement Ardunio Uno almost anywhere at half the price of a Mach3 breakout board. (I am not even mentioning that Mach3 needs to be purchased while GRBL open source) The second part is complexity, GRBL is straight forward and does everything you need from a machine, ideal when you consider 90% of our clients are experiencing CNC machines for the first time.