[devit]

Because an unexploitable browser (which is the main fundamental advantage of Servo) is more important for Tor than elsewhere since an exploit will compromise anonymity.

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.