But why not just C/C++?

MicroPython lets developers write and test code much faster. Python’s syntax is simpler and more readable than C/C++, and the MicroPython firmware already includes everything needed for development. You can upload a script and run it immediately, or control the hardware live by typing commands, without recompiling and re-flashing the firmware after every change. This also makes updates simpler: changing your code only means replacing .py files. It also has a universal API across many boards.

But that convencience comes with the cost of lower speed, higher memory usage and less predictable timings. so pick the one, right tool.