It's a lot easier to swap out the renderer or the CSS engine for a new one than it is the whole core of the browser engine. Mozilla decided that replacing Geck

It's a lot easier to swap out the renderer or the CSS engine for a new one than it is the whole core of the browser engine. Mozilla decided that replacing Gecko as-is was not reasonably likely to actually happen, and that further efforts towards Servo would be better made by continuing to evolve Gecko.