Because it is not web based.

Does that matter? I bet the core Servo libraries don't even deal with networking. And using Servo has obvious advantages such as a being a good render engine supporting incremental updates, probably being well-documented, well-tested, and conforming to well-known standards (yes, also used on the web, but does that matter really?)

Honestly, it matters. Current web technologies aren't the end game solution for interfaces and, IMO, are pretty horrible to work with for that purpose. (Pretty beaten up topic by now, but lets take a moment to think for a second about vertical centering in HTML+CSS. /sigh </rant>)

Still, my answer was just referring to the fact that this is not using web tech, and thus, not using servo or any other existing browser engine.

flexbox + viewport units have made this better.

Because it is not HTML based.