Why is the (American) standard of beauty so often associated with just weight?

Lately, I’ve seen this picture of Marilyn Monroe floating around the internet usually with a caption that says something about how she was considered sexy in her day and now stick-thin is considered sexy and ain’t that just horrible.

However, there are still other standards of beauty in this country that no one worries about changing. What about race? Body type beyond mere weight? Height? Coloring?

Why is the standard of beauty so often associated with weight when there are so many other factors?

Yes, I do know that there are super models who are not white, and not blonde-haired and blue-eyed, but I have observed that white and blonde is still more often considered the standard of beauty in this country.