I suspect the answer is because of compatibility. Chromium has become the industry standard, everyone tests only on Chrome (and Safari for mobile) so you can expect fewer issues if you're based on chromium.
But I think it's a valid question. Because for a niche browser (which most of these alternate projects are), mainstream compatibility for all websites doesn't seem like it would be the most important thing. I think the idea of using Gecko[0] is a solid one. :P ;) xx