You might not know what features you really want and writing it in microPython lets you try it out faster. I think that the chip they eventually might run it on might also be different.
As far as MicroPython goes, same thing. It's great for prototyping, but it's still somewhat hackey, slow, not that memory efficient. It doesn't need to be for its use case...but for production ready sensors I need to be sure to work the best they can off the shelf, I'm gonna write that in C/C++ or most likely Rust (if there is an LLVM backend for the chipset).