-Lived over 50 years with ALS, after being given only 2 years to live when he was in his early 20's. This has also helped him to maintain a strong public image.
-Revolutionised our understanding of the Big Bang, Black holes, and many other areas of cosmology, effectively showing the universe started as a space time singularity.
-Discovered Hawking radiation.
-Wrote A Brief History of Time, which was the best selling book for an extraordinarily long time and has sold over 10 million copies, becoming the first science book to really capture the public interest.
-Interesting/controversial comments about God, AI, and many other topics.
-A number of films/books/documentaries about him.
-Featured in Star Trek, The Big Bang Theory, The Simpsons, etc, so has good public awareness.
In short, google him to find out!