One BIG Question is Why this forum use Ruby instead PHP (maybe Yii self)?

Yii is great for speed and build REST API service. But when I start learn Yii and open this forum, what! this use Ruby (Discourse). Why not build with Yii to?

Please give me professional answer.
Thanks.

Maybe because Discourse is a ready forum product?

Building a good forum engine isn’t a simple task that would require huge effort and resources.

We’re focused on building a framework, not building a forum engine.

If there will be a forum engine built with Yii that is maintained by someone else and is as convenient as Discourse, we’ll be glad to switch from Discourse to it.