The good Doctor created a world in which multiple generations of readers have found joy and solace. That is the accomplishment of a great writer, never mind the length of the text or his gift for rhyme. The style and prose of Dr. Seuss' work has influenced countless authors that followed, the highest tribute to greatness that can be afforded to a writer.
Dr. Seuss wrote simple books for children, which facilitated learning of the English language. Many parents still use his books today to get their children to start reading. Dr. Seuss's stories are fun and engaging, with repetitive rhyming and short words, while also teaching countless American children about the joys and benefits of reading.
When asked to name some of the greatest writers in American history, most people probably think of authors like Ernest Hemmingway, F. Scott Fitzgerald, Mark Twain and William Faulkner, but Dr. Seuss should absolutely be included amongst these names. Dr. Seuss has written so many stories that are treasured by people of all ages. His works are full of creativity and wonder--two things that are far too scarce in the world today. His books are absolutely children's classics, though their cleverness makes them just as beloved by adults. In addition to his success of as an author of books, many of Dr. Seuss' works transcended their original book format to be made into movies, plays and musicals, not to mention it's rare to hear a graduation speech devoid of a Dr. Seuss quote.
Dr.Suess is a wonderful children's author, but he does very little to explore anything beyond funny characters and plays on words. Great writers explore deep emotional issues in our human nature. Dr. Seuss does not write real human characters and his books are relatively free of conflict. This does not create the tension and realism that we see from history's greatest authors.