Here is the thing. If you enter domain.com into the address bar of your browser, your browser will always go to the http site unless you do HSTS preloading. Your first visit to a website is not secured by https. So lots of people do a http -> https redirect, which means you can do a man in the middle attack on the http port and the HSTS header will never get loaded in the first place. https is significantly less effective than it should be.