Hi,
it is generally ignorance. Also people like to believe eating disorders are just something women go through, another fad diet so to speak, they don't even take into account that it's a mental condition at all or that it could be covering a multitude of other issues.
People often make a joke of issues they are uncomfortable talking about or are uncomfortable accepting are a real problem and over the years most mental health problems fit this category - they are not only joked about but also taken lightly and things are said in vain! how many times have you heard someone say " i'm depressed, or gosh i would be better of dead, i'm going crazy, you belong in the nut house, if you don't eat something you'll turn into a skeleton, your a psycho...the list goes on and on!!
I would say annorexics get as much flack really, although I guess bulimia is seen as not as serious simply because most bulimics actually can be normal weight etc (stupid reason I know) so basiclaly if you look normal theres nothig wrong with you and many people can take that stance on things!
I will still say the main issue is ignorance because if the people making the jokes knew how it was to be bulimic or to have any mental health issues I don't think they would say some of the things they do.
In real life some people joke about their conditions and stuff as a way of coping or putting on a front to others, I know I do it...so really there can be various reasons
best of luck
xx