It is often erted that Spin is a uniquely Quantum Mechanical phenomena, why?

Because it isn't really spin. Orbital angular momentum doesn't need a clical og, it is clical. Electron spin is not orbital angular momentum. Every electron has the same value of 'spin' which can't be changed. It probably would have been less confusing if they named this quantum parameter in the same manner they named quarks, like up, down, strange, and charm. You know they are meaningless names and you don't try to picture a particle matching the name.