Ladybird is a browser, servo is a rendering engine. They have different aims. Some of Servo's components are developed by Mozilla for the use in Firefox.
The first serious implementation of browser using Servo started very recently in the form of Verso.
Is ladybird not using a custom rendering engine too? Or you mean they're following existing techniques more whereas servo is focused on research over production?
One of Servo's aims is to make an embeddable rendering engine; an alternative to chromium. Chrome/ium is not just a browser it powers everything nowadays, apps, all major alternative browsers run chromium underneath. And that's a huge problem.
The ecosystem of Servo is very very different. It's very modular and many components are fast moving targets developed by multiple companies. While with ladybird it's a comparatively singular thing (developed mostly by a single guy).
The embedding is a serious and ambitious effort and possibly one of the reasons why Mozilla dropped it.
It has to support vast array of ecosystems and compete with a very mature and widespread electron/chromium to cover the same use cases.
It's a massive effort, but is already being experimentaly integrated by companies (like embedding into Qt toolkit).
And to simply put, Servo has a much larger scope, their aim is much wider than making a browser. The browser will be one of its use cases (It's already being developed intensely in the form of Verso by different team).