A lot of it has to do with the location. The short description is more versatile than the long description. The long description displays in one spot, the bottom of the product page. The short description displays in several spots, it can be displayed in any product-list, category pages, on the product page, ect. For flexibility, it could be changed to be in a div tag, but it would require people to update their themes. Also, it does help keep a site consistent. People tend to mark things up and use large fonts when allowed and it makes category pages look bad.