No science started out as a "hard science". Psychology is hard to quantify yet, because our currently available options for measurements are insufficient.
? That would still be biology and therefore reducible to chemistry and physics.
The approach of "everything is reducible to physics" is actually a philosophical theory that tries to describe what is reality. Is the material world everything that exists? Or are our thoughts (our knowing of things) actually a different reality? Etc.
In the end, the differentiation into the different sciences is simply an aid for people. I wouldn't pay it that much attention because it really doesn't tell you anything.
Being reducible is part of it, but I think reproducible is more important. Psychology is not reproducible. You can get statistical equivalents, but not exact reproduction of results.
I think being reducible is all of it. Even if it's reproducable you can know THAT something is true, but not WHY it's true. I think the why, or at least the ability and intention to get there, makes something a hard science.