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.
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.