It’s rather uncommon that you consider TOR support to be a higher priority than “a browser that works”, so I’d like to hear more about the ordering of your first two priorities, specifically.
Also, since Tor browsing is often done with JavaScript off, and removing JavaScript removes a huge amount of web features that need to be supported (including a JavaScript engine, which I think Servo still doesn't have a pure Rust implementation of), it's much easier to make a usable browser for this use case than others.