Need to give your event a boost? The legendary Eddy Merckx was once asked why the Tour de France was the pinnacle of a racer's cycling career. His response was, "there is only one, you know.
That explanation may seem lacking, but it speaks volumes as to the popularity of Le Grande Boucle for both racers and fans. Every July, the eyes of the cycling world are squarely trained on France, and year after year the Tour provides memorable moments—most good, a few bad--that keep everyone on their seats wanting more.
What was Merckx talking about? There are literally hundreds of races each year on the professional cycling calendar at locations all over the world. Unfortunately, because there are so many events, most races overlap with another race, usually in another country, leaving teams with the choice of where to go.
The Giro d'Italia, the most important race in Italy, is held in mid- to late May. At the same time, the Vuelta a Catalonia, arguably the second most important race in Spain, is also taking place. Rules to Ride by at the Tour de France.
Unlike spring classics such as the Tour of Flanders and Paris-Roubaix, which require consummate one-day racing skills and overlap with stage races in Spain and Italy, the Giro and the Vuelta a Catalonia are top-flight stage races in their own right and are favored by anyone with Tour de France aspirations. The result is a dilution in the fields. While there is still great racing at the Giro and at Catalonia, the best of the best are not necessarily competing mano-a-mano.
The racing calendar in July is totally dominated by one event, the Tour, which means that the best riders in the world will be there ready to ride.
This brings us to the next reason why "there is only one, you know. Most of the other racers—domestique duties excepted--are there only for "training.