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
I agree it's a valid question and it seems my suspicion is not the main reason. Based on the other replies to the parent it seems that choosing Gecko requires more work.
[0]: https://github.com/mozilla/gecko-dev