I think a lot of people are missing how precise you are being in your phrasing and think that you are making objective claims about prestige and importance within the sport.Setting aside the differences between "greatness," which I thought was the matter at hand, and "prestige," I'll try to answer as if you'd asked about both.
History, both recent and otherwise.
Panache and flair.
Road and weather conditions.
Unpredictability (this, along with History, is probably the "biggest" reason).
Percentage of riders at the start who are racing for the win.
Generation of anecdotes, photographs, and other cultural artifacts that speak to all of the above.