The UCI calendar offers three Grand Tours, but the Tour de France really stands alone in terms of global interest.
This chart shows how global searches using the term "cycling" always peaks in July when the Tour is held:
It also shows a general decline in interest from peak years in 2004.
Most of the world isn't that interested in cycling. In the last decade Australia, Scandinavia, and South Africa joined Europe and North America in curiosity about the sport:
In 2012, the British Isles suddenly became the most cycling-mad (at least on the Internet) region on Earth. This is the effect a single rider (Bradley Wiggins) and a single team (Sky) can have on a country.
The UCI has also noticed the relative lack of interest in cycling in Asia, which is why Pat McQuaid has made such a noticeable push to develop races like the upcoming Tour of Beijing.
It's really not the location of a race that drives interest, though--it's the nationality of winning cyclists. Just as Wiggins inspired interest in British cycling, Lance Armstrong (and Greg LeMond before him) brought in a North American audience.
Notice, both of these bike racers excelled at the Tour. Andy Hampsten's Giro victory goes in the same category as Ryder Hesjedal's win there--a footnote, at least in terms of popular interest.
That's because the Tour dwarfs the other Grand Tours:
Consider four top riders from different regions: Ryder Hesjedal, Tejay Van Garderen, Vincenzo Nibali, and Joaquim Rodriguez. Interest in Hesjedal and Van Garderen is consistently greater than interest in Nibali and Rodriguez, although Nibali and Rodriguez are consistently better bike racers:
No one outside Italy is interested in Nibali:
Just as no one outside of Spain is interested in Purito, and the same being true for Hesjedal and Van Garderen.
The truth is, we follow our countrymen, and this is what drives the popularity of global bike racing. The same is true, of course, on a smaller scale--we follow MABRA racers doing national or international races; we follow our own teammates; we are more interested in our own racing than in others.
Where the contest takes place is relatively unimportant, it seems. Who is competing--and particularly who is winning--draws interest. When the first Chinese or Indian rider wins the Tour de France, that's when the sport will move from a parochial to global sport.
Soccer's World Cup achieves this partly by the nature of the tournament, which guarantees regional representation--if China doesn't make the tournament, at least neighboring Korea does. That the teams are national is, I think, less significant than the presence of nationalities.
Holding races in China for largely European teams won't inspire interest in bike racing there. Reforming team structures to encourage the development of Chinese cyclists will.