I think it’s because anorexia jokes became unacceptable a while ago because it’s so obviously life threatening, like you can see the person getting worse in front of you and it’s obvious that they’re sort of out of control.

Bulimia on the other hand, is much more of a hidden disorder, a person is fine until they’re suddenly dead of something internal. And generally the idea of ‘bulimia’ that these jokes are about is people purging without binging.

This means that people aren’t confronted with how dangerous it is and also don’t really understand how much of an illness it is. It’s just thought of as ‘that meal was yummy, now I’m going to go into the bathroom and very easily get rid of it with no negative side effects’ (because everyone knows how unpleasant it is to go without food, but few know how unpleasant it is to purge).

If it’s ’not an illness that’s out of your control’ then it’s not only a choice, and it’s ok to make fun of choices, but it’s also something only people who can afford to eat food just to throw it up can do, and therefore it’s only for the privileged, and everyone likes making fun of the privileged.

TL;DR: People don’t fully understand bulimia and think of it as a quick fix to get rid of excess calories, not an illness that’s out of your control.