Most other parts of Servo were not mature enough to integrate at the time Mozilla decided to end support for the project and didn't look like they would be mature enough any time soon. The DOM engine for example was in the early stages of being completely rewritten at the time because the original version had an architecture that made supporting the entire breadth of web standards challenging.
Keep in mind that you can continue adding Rust to Firefox without replacing whole components. It's not like Mozilla abandoned the idea of using more Rust in Firefox just because they stopped trying to rewrite whole components from the ground up.
Mozilla laid off the full Servo team, but never publicly announced this afaik. Wikipedia includes it here: https://en.wikipedia.org/wiki/Firefox#cite_ref-120
Ladybird, by contrast, is a developer-lead open source project that has no such constraints. They also don't have a product yet but I'm sure the picture will be radically different in a few years.
Conway's law in action.
Not once in my career have I come across a problem that wasn't cultural. There are no purely technical problems in software. Everything can be achieved, everything can be worked around. All one need is a consensus. Enters cultural problems.
> The managers want to keep their jobs more than they want Firefox to succeed.
Coincidentally, also throughout my career, not once have I met an engineer that didn't put the entire blame on managers. Introspection really isn't our forte, is it? :)
Only recently when it moved over to the Linux Foundation has Servo started being worked on again