Almost every consumer 3D printer out there runs on top of Arduino. Sure that’s a small market compared to all electronics. But it’s still a big market in absolute terms.
This was true three years ago. Plenty of 3D printers are now using ARM. The Monoprice MP Mini Delta has been out about two years now with a price of $175 US. Marlin 2.0 has support for ARM, I think it would be accurate to say "Almost every consumer 3D printer out there runs on top of Marlin", although there are a few alternatives like Smoothie.