The big reason was that Mozilla ended embedding Gecko with Firefox 4/Mozilla 2. This made it nearly impossible to run Gecko in anything but an XUL shell. I understand the reasons cited at the time, largely available resources, but it was very short-sighted and I think it hurt Gecko's mind shares in the long run.
I loved Camino as well. A truly excellent browser.
Gecko ending embedding is almost certainly a major reason why everything built with a browser engine is either a Chromium wrapper (not true embedding, but no browser chrome) and WebKit (true embedded).
It’s a real shame. Gecko is a nice engine but XUL is a much harder sell.
Correct. It's not intended as a Gecko replacement, and several important components in Servo are already part of Gecko. Servo will continue to evolve and new features will originate from it, but it lacks a lot of the legacy and quirks rendering (intentionally) that would be required to make it a full-fledged engine of its own even if Mozilla wanted it to be.
I loved Camino as well. A truly excellent browser.