Servo isn't a JS engine. Do you mean why didn't they abandon their mission statement of developing a truly independent browser engine from scratch, abandon their C++ code base they spent the last 5 years building, accept a regression hit on WPT test coverage, so they can start hacking on a completely different complex foreign code-base they have no experience in, that another team is already developing?

Ladybird has a strong "all dependencies built in house" philosophy. Their argument is they want an alternative implementation to whatever is used by other browsers. I'd argue they would never use a third party library like servo as a principle.

Well for one, Servo isn't just JavaScript, it's an entire engine. Closer to Blink & Gecko.

Secondly, Ladybird wants to be a fourth implementor in the web browsers we have today. Right now there's pretty much three browser engines: Blink, Gecko and WebKit (or alternatively, every browser is either Chrome, Firefox or Safari). Ladybird wants to be the fourth engine and browser in that list.

Servo also wants to be the fourth engine in that list, although the original goal was to remove Gecko and replace it with Servo (which effectively wouldn't change the fact there's only three browsers/three engines). Then Mozilla lost track of what it was doing[0] and discarded the entire Servo team. Nowadays Servo isn't part of Mozilla anymore, but they're clearly much more strapped for resources and don't seem to be too interested in setting up all the work to make a Servo-based browser.

The question of "why not use Servo" kinda has the same tone as "why are people contributing to BSD, can't they just use Linux?". It's a different tool that happens to be in the same category.

[0]: Or in a less positive sense, went evil.

Because they're not servo and servo is still in the race. Merging those projects is against making an independent browser(s)

NIH syndrome