First, this is optional software that almost every public server in the world is currently not using. It's like saying just because executable whitelisting exists that downloaded exploit payloads on operating systems is no longer a threat. If nobody uses it, the threat still exists.
In addition to actually implementing it on your server, every single user still has to make a secure initial connection over a trusted network on every device they'll ever use to get to that site.
But not every client even supports HPKP. There's lots of older software which doesn't support it, and IE doesn't support it at all, which by itself would leave 12% of all clients vulnerable.
