Why is curl using HTTP/1.1 rather than HTTP/2 on mac?

According to this https://curl.haxx.se/docs/http2.html

Since 7.47.0, the curl tool enables HTTP/2 by default for HTTPS connections.

Installing the latest version with homebrew and checking:

curl --version
curl 7.54.1 (x86_64-apple-darwin15.6.0) libcurl/7.54.1 SecureTransport zlib/1.2.5

but running curl on an HTTP2 enabled URL (e.g. test with https://tools.keycdn.com/http2-test) I get:

curl -I http://www.google.co.uk 
HTTP/1.1 200 OK

and

curl --http2 -v http://www.google.co.uk
curl: (1) Unsupported protocol

Any idea why it's using HTTP/1.1 rather than HTTP/2?