Ask HN: why are requests pushed to heroku dynos?

The last days saw numerous discussions on HN on intelligent vs random routing on heroku. One thing that I don't understand is why requests are routed to dynos when they are entering the heroku mesh, and in the case of random routing might get stuck behind a slow request.

What I don't: why are requests pushed to dynos, and why doesn't a dyno actively pulls a request from some kind of shared queue after it is done processing the previous request.

Can someone explain this?