Because unless you want to expose your Ruby server directly to the internet without a load balancer, there isn't much to gain in having HTTP/2 between the LB and the app server.

I'm curious though, what's your use case to and HTTP2 up to the app server?

I think it's in the works for Puma.

Tenderlove posted this gist a while ago as proof of concept:

https://gist.github.com/tenderlove/d68d9a3c4f7941192c9b

Mayu uses HTTP/2. It's still experimental though. https://github.com/mayu-live/framework