Here is the Library of Congress’s reasoning for why they selected Shrek:
Even by DreamWorks standards, the charm and magic of Shrek seemed extraordinary upon its initial release almost 20 years ago — and its power has yet to diminish in the intervening years. With this story of a green-skinned, solitude-loving ogre, Shrek, who embarks on a noble quest, alongside his new friend, a lovable donkey, the film manages to be both a send-up of fairy tale tropes and an affectionate tribute to them. Entertaining and emotionally impactful at levels to be appreciated by both children and their adults, Shrek was a mega-hit upon its release and has been followed by three equally enchanting sequels, a TV holiday special and a Broadway adaption. Mike Myers, Eddie Murphy and Cameron Diaz lead the strong voice cast.
Among the 800 films that are now on the list, animation represents a pitifully tiny fraction of the works. The films include Who Framed Roger Rabbit?, Pinocchio, John Henry and the Inky-Poo, The Old Mill, Luxo Jr., Bambi, Steamboat Willie, Duck Amuck, What’s Opera, Doc?, The Story of Menstruation, The Lion King, and Gerald McBoing-Boing, among a smattering of others.
While most of the animation works on the list are worthy of inclusion, they don’t come close to representing the vital impact that animated film has made on American culture, a contribution that far exceeds the influence of many of the hundreds of live-action works that are included on the National Registry.