The problem is, basically, middleboxes. It's unfortunately still common to intercept HTTP connections and redirect them to some sort of caching proxy, and some

The problem is, basically, middleboxes. It's unfortunately still common to intercept HTTP connections and redirect them to some sort of caching proxy, and some of these proxies will misbehave when seeing HTTP/2 on the wire, causing hard-to-diagnose connection issues. It's even worse when firewalls are involved, since they can get confused easily and will drop the connection (manifesting as a hang in the browser) in that case. With TLS, not only is it less common for the connection to be intercepted, but also a MITM proxy which doesn't understand HTTP/2 won't negotiate it with either end, transparently falling back to HTTP/1.1. And firewalls will only see the outer TLS protocol.