The only reason is network effect. Most "shields" and devices are tested and libraries made easily available for Arduino/Atmel 324P. Going to ARM means having

The only reason is network effect. Most "shields" and devices are tested and libraries made easily available for Arduino/Atmel 324P. Going to ARM means having to rebuild some of your toolkit, or finding it. But again, it's ARM. And the "more expensive one" is 32bit.