Why isn't it okay to make fun of cancer, but it's okay to make fun of other medical conditions?

I often see people making fun of people with psychiatric, neurological disorders and seizures specifically in the neurological category, but when someone makes a joke about cancer or something it's a serious deal? I'm not trying to say cancer is any less, I was just wondering why? Disorders like anorexia nervosa are completely deadly because it is also a slow death. I was just wondering why. Seizure disorders are horrible, too (I'm epileptic) and people make fun of them by faking a seizure by like twitching their arm and making weird noises. Is it because people think that people with psychiatric disorders and personality disorders are psycho/crazy/etc.? If that's the case, people really need to be more educated about this.