because a JPG is displayable in any browser with a simple click...
a PDF, on the other hand, may be handled in vastly different ways, depending on the browser and its settings. Some may have to have the browser load a plugin (time-consuming), some may have to save the file to their local drive, navigate to the file, etc... and open it with whatever PDF viewing program they use (pain-in-the-butt)...
as stated, JPG is single-click viewing with little or no waiting...