The ATmega 328P an Arduino Uno or Nano uses has 2KB of memory and 32KB of flash storage for the program. Having some sort of micropython interpreter there would be impossible, and even if it could be achieved, somone still has to write the low level C/ASM code to make it all work. For many embedded systems, low level languages like C or C++ (perhaps Rust for a lot of ARM micros) is the only sensible choice. If it's not a multi user system that's connected to a network, people trying to abuse memory unsafe code isn't a concern.