A combination of two factors that are rather specific to myself. One being that even tho its quite some time ago i have a bit of c++ experience, while i never tried to code C#. So when i got the idea to try to run the heavy simulation in an "extension" it was just a language i was a bit familiar with. Maybe C# would have been the "smarter" choice, i just never digged into it yet.

The other reason is that in my well closer circle i got quite some people that code c++ on a daily basis and i don't know (at least i think so) anyone doin C# actively, so i thought if i would get stuck i have people i can easily ask about my c++ problems.

So it wasn't a "i think c++ is better than c#" decision rather a what do i know and what resources i have available easily.

C# is a "first class" language in Godot and doesn't need a GDExtension, but it's not compatible with web builds yet. There is a draft PR that includes C# support but it adds about +50 MB to a web export and is also missing a few features.

Not sure how relevant this is for this simulation use case but IIRC C# has a bigger overhead when communicating with the core engine compared to GDScript or C++.