Two thoughts:
1. The intercontinentalism of the "competition" means that spectators get almost 24hr coverage for 365 days (God willing). Knocks even a five day Test Match into a cocked hat in the VFM stakes.
2. How will the riders avoid going insane? On the rare occasions when I've had to spend long periods in the saddle alone, my brain goes into some weird loops and breakaways. They'd be carting me off in a straight jacket by day 5 (although, unlike these riders, the men in white coats wouldn't have to travel far to get me).