I'm not sure that graph shows what you think it shows.

That graph shows that there's an obnoxiously large number of websites that don't support something that there's zero reason not to support.

It costs $0 and takes maybe half an hour to implement, browsers are screaming "not secure" to each visitor, and one in five websites still don't support it. "Make websites faster", on top of being vague, is way more difficult to implement.