Building another rendering engine is too much work. We did that at Opera and it was the best engine around, but sadly Opera killed it after I left. There is a lot of complexities there that most would not understand.
There is not a lot of alternative engines out there and I do not think it would be worth the effort to try to be compatible with another engine. We modify Chromium. It is a lot of work to maintain that. Doing that with more than one engine is just not feasible and would not bring us anything.
If we treated the rendering engine as a black box, we might have been able to do more, but that is not what we do.