Why Rust? Because Rust has excellent concurrency support and memory safety, both of which are useful for databases.
For something as low-level and performance-intensive as a database intended to replace MySQL/Postgres, Go wouldn't work because it's garbage collected.
I feel Rust is still not old enough to be used on databases. It is a relatively new language with an amazing future. But it might not be a good choice for a new product that is meant to be the workhorse of a system. C++ is the correct and safe choice under the circumstances. Good question though. Its always beneficial to question architectural choices in order to see the other side of the coin. :)
It might be because I was watching MemSQL for a long time, but I think Brodlist is probably years away from being used in production. Rust is stable now, so I think there's an acceptable tradeoff between the risk of Rust lacking an ecosystem and the benefits of using Rust.
Anyway, it doesn't matter that much in the end. I was just curious.
For something as low-level and performance-intensive as a database intended to replace MySQL/Postgres, Go wouldn't work because it's garbage collected.