Let's also say that you want to be able to hire programmers to write and maintain it. And it would be nice to make it open to many more people so they could fin

Let's also say that you want to be able to hire programmers to write and maintain it. And it would be nice to make it open to many more people so they could find and fix bugs. After that, what's left? At the moment, none of the above languages have any significant talent pool to draw from. If you're operating at "Twitter-scale" (I guess that's now a thing), you also need to be able to spin up a team of people that can get to work quickly without having to learn a language as they go. That eliminates a great number of languages just for logistical reasons. Rust might get there (and I hope it does!), but it's not there yet. Maybe Twitter already has a significant number of skilled Java engineers, so they decided to use what they already knew really well. There's nothing wrong with that. Choosing any of the other languages you listed would have been a far riskier strategy.