Why? I'm curious, what makes Cavill so "Superman" for everybody? Outside of possibly the first half of Man of Steel and maybe a scene late in Justice League, there is nothing that makes him stand out. When I think Henry Cavill, I don't automatically think "Superman." Christopher Reeve, though, will always be Superman first. Cavill just ain't there yet.
Hell, Momoa is more identifiable as Aquaman than Cavill is as Superman.
For me it's his size and look. When I see him I see Superman, not someone playing Superman. If he had been giving a script like Donner's Superman instead of Snyder's dark mess I think people would see him the way they see Reeve as Superman.