Likely a joy for Rust and a desire to try something ambitious that hasn't really been done.
Rust doesn't have the mature libraries that C# offers, but it isn't barren anymore either. It does offer nice assurances like entirely avoiding race conditions by default (when using safe Rust). Rust often leads to performant code, but it's hard to say if that's really a feature of the language or just a bias due to the kinds of people using it and the projects they work on.
If you're just looking to get into game development without any other goals in mind, then C# is a safer choice
Rust doesn't have the mature libraries that C# offers, but it isn't barren anymore either. It does offer nice assurances like entirely avoiding race conditions by default (when using safe Rust). Rust often leads to performant code, but it's hard to say if that's really a feature of the language or just a bias due to the kinds of people using it and the projects they work on.
If you're just looking to get into game development without any other goals in mind, then C# is a safer choice