The reason "retard" is more offensive to people with mental disabilities (even when not used directly as a slur) is that its use in that context is more recent

The reason "retard" is more offensive to people with mental disabilities (even when not used directly as a slur) is that its use in that context is more recent than the use of "idiot" or "imbecile". That is all. Really. Give it a few decades and we wouldn't be having this discussion because no person with mental disabilities would feel offended by anyone using it as a mere synonym for "dunce". Of course so-called Internet feminists might still demand your head on a silver platter even then, but (in stark contrast with actual feminists) nobody should be paying attention to them to begin with, even if they make it hard not to.