Interesting question -- googled this which answers it pretty well:
Additionally, unless the slogan is in itself inherently distinctive and qualifies as a mark in itself, the USPTO requires that the slogan be identified with the product or service so that the consuming public, upon hearing the slogan, relates it to the particular product or service. This requirement means that the slogan has developed what the law refers to as a “secondary meaning.” The best examples of this type of slogan are “Just Do It” (Nike) and “Finger Lickin’ Good (KFC).
Additionally, unless the slogan is in itself inherently distinctive and qualifies as a mark in itself, the USPTO requires that the slogan be identified with the product or service so that the consuming public, upon hearing the slogan, relates it to the particular product or service. This requirement means that the slogan has developed what the law refers to as a “secondary meaning.” The best examples of this type of slogan are “Just Do It” (Nike) and “Finger Lickin’ Good (KFC).