When MIG was started, in the summer of 2013, Rust was still too unstable to be a good candidate. Go, however, already had the toolkit to build cross-platform agents, so I went with this.
18 months in, I still think it was the right choice. Go is extremely powerful and yet simple enough for contributors to pick it up in a matter of days. I don't think we could have delivered as much value going with Rust, it's still too young and lack the necessary libraries (AMQP, OpenPGP, etc...).
Mozilla is a large and diverse company, not everyone must write in Rust.
Also it could deal with too many unsafe libs, so it being safe wouldn't be as beneficial. That's the reasoning why IIRC homu was chosen in Python instead of Rust.
