The internet doesn't just consist of Firefox and Chrome. There's mail servers, mail clients, FTP clients, etc. And Firefox and Chrome can be behind middleboxes

The internet doesn't just consist of Firefox and Chrome. There's mail servers, mail clients, FTP clients, etc. And Firefox and Chrome can be behind middleboxes and/or personal firewalls that only talk TLS 1.1 on the public-facing side. And finally, there's a lot of android devices running Android < 5 which all don't support TLS 1.2 and never will.