I can think of many reasons.... "we were using python to quickly explore the space and rewrote to rust once we figured out the interface and started to focus on speed and safety", or "the original authors wrote this software in smalltalk but we'd rather it be golang because this simplifies deployment"

> You prefer a changing codebase over something that has evolved and is being tested over a long period of time?

As long as the test-cases are there, why would I care what language software is written in?