There isn't a specific feature of Flux that requires a new language, although part of the language is a query planner and optimizer that is tied to certain functions in it. Technically any Turing complete language is equivalent, but in practice that is seldom true. Performance is one vector, but so is expressiveness, aesthetics, ease of use, etc.

We chose not to use Lua because it's not a widely known language. Instead, we chose to create a language that looks and feels much like Javascript, which I think is the most widely used language today. However, we wanted to limit the scope of it and to add new syntax over time to make frequent tasks easier to express.

Any time you create a new API or library, you create new surface area for a developer to know. A language is no different than this. Some languages are also much easier to learn than others. For example, I found Go a language that I could pick up the basics in very quickly, while Rust is something that I'm still learning over months of effort. I like both languages, but their learning curves are very different.

I think Flux is quite easy to pick up for many programmers, although I'll have to see questions and have countless interactions with people trying to learn it to prove that out and make adaptations over time.

Finally, one goal with including the UI in InfluxDB 2.0 is that as we improve it, most users won't have to learn Flux at all. They'll be able to accomplish what hey want by just clicking around the interface. We're not there yet, but it's what we aspire to. And we want to have control over the language to build it in conjunction with UI tooling to automatically manipulate it.