Developers who work on git think it will help them do their jobs better. Do you need any more reasons beyond that? They don't need to justify it to users necessarily.
There's also the fact that if you want to recruit systems programmers for a project like git, the 19-year-old catgirls who are likely to be interested in that sort of work all work in Rust. Ask one to hack a legacy C code base and she might nyao at you angrily >:3
