These days "microcontroller" refers to a part that has self-contained flash and SRAM, and the MMU/Linux-capable SoCs need external mass storage and DRAM to run.

These days "microcontroller" refers to a part that has self-contained flash and SRAM, and the MMU/Linux-capable SoCs need external mass storage and DRAM to run. So there's a cost adder on the Linux parts. You might also need a denser PCB to handle DRAM signals (although Jay Carlson might be right on this).