Because of the use case. Go wins if all you need is the easiest way to write services with high concurrency requirements. I expect this is true for Twitch's systems.

Crystal is still immature, Rust is more suited to use cases where you want to avoid garbage collection.

Hype is not the only factor but it makes hiring easier. And anything Google puts its weight behind will get hyped. More often than not it's better to choose a technology which suits your organisational (read: hiring) needs.

I'd argue that Java or C# would have worked out just fine for Twitch. There was a recent post on Twitch's early architecture, and it seems they started out with Ruby. Unsurprisingly, they had to switch from it once they needed performance (similar story happened with Twitter).