Also, Menchov was 5th 2008 Giro 4th 2008 Tour.
Anyway, the main reason is that the Tour péloton is significantly faster than either the Giro or Vuelta. Part of that is due to the better conditions than there often are at the Giro owing to time of year (more chance of rain, wind and other factors that slow riders down) and part of it is due to other weather factors (who the hell wants to ride 50km/h+ in 35º+ heat of a baking hot stage through Andalucía with no breeze and no shade?), but part of it is simply that the Tour is so important both to riders and, more importantly, to sponsors, that the fight to be in breakaways, the fight to keep the bunch going, the fight to be where it matters to get the exposure is more intense so it drives the speed of the péloton up. Also, the Tour's backloading tendency in recent years makes the péloton a tenser place because everybody still has something to protect.
Another factor is that with the weather as it often is at the Giro - 2000m+ altitude with the snow on the ground - the chances of getting sick are increased. It's a lot harder to win a GT and recover in a month for another one when you're sick; the Tour winner will have a media blitz and the post-Tour crits, but they're all fixed and for show, you can phone it in. The Tour and Vuelta are typically held in much better weather (also, that suits a few of the riders who've attempted it recently, such as Froome).
Realistically, staying at the business end to win a GC of a GT by the time the Vuelta rolls around in September means that freshness is a factor, and going from GT to GT obviously limits that. The people who've done good GCs at two consecutive GTs in recent years have been those who've been in a relatively comfortable position in the first GT (Froome in 2012, with Sky's 1-2 so clear and comfortable and with the team clipping his wings, and Sastre in 2008, who was able to bury himself in the pack for most of the race with Fränk Schleck up the road) or those who have always been strong all season long (Valverde, Evans). Also, both Sastre and Evans were helped by the 2007 Vuelta being one of the easiest to maintain form through; the toughest mountaintop finish was on stage 4 after few other climbs and apart from El Purche most of the other summits were grinders like Arcalis. As long as the Vuelta keeps going with the carnival gradients preserving time will be harder (plus, the skillset needed at the sharp end of the Vuelta starts to differ ever more from that needed at the Tour).
Anyway, the main reason is that the Tour péloton is significantly faster than either the Giro or Vuelta. Part of that is due to the better conditions than there often are at the Giro owing to time of year (more chance of rain, wind and other factors that slow riders down) and part of it is due to other weather factors (who the hell wants to ride 50km/h+ in 35º+ heat of a baking hot stage through Andalucía with no breeze and no shade?), but part of it is simply that the Tour is so important both to riders and, more importantly, to sponsors, that the fight to be in breakaways, the fight to keep the bunch going, the fight to be where it matters to get the exposure is more intense so it drives the speed of the péloton up. Also, the Tour's backloading tendency in recent years makes the péloton a tenser place because everybody still has something to protect.
Another factor is that with the weather as it often is at the Giro - 2000m+ altitude with the snow on the ground - the chances of getting sick are increased. It's a lot harder to win a GT and recover in a month for another one when you're sick; the Tour winner will have a media blitz and the post-Tour crits, but they're all fixed and for show, you can phone it in. The Tour and Vuelta are typically held in much better weather (also, that suits a few of the riders who've attempted it recently, such as Froome).
Realistically, staying at the business end to win a GC of a GT by the time the Vuelta rolls around in September means that freshness is a factor, and going from GT to GT obviously limits that. The people who've done good GCs at two consecutive GTs in recent years have been those who've been in a relatively comfortable position in the first GT (Froome in 2012, with Sky's 1-2 so clear and comfortable and with the team clipping his wings, and Sastre in 2008, who was able to bury himself in the pack for most of the race with Fränk Schleck up the road) or those who have always been strong all season long (Valverde, Evans). Also, both Sastre and Evans were helped by the 2007 Vuelta being one of the easiest to maintain form through; the toughest mountaintop finish was on stage 4 after few other climbs and apart from El Purche most of the other summits were grinders like Arcalis. As long as the Vuelta keeps going with the carnival gradients preserving time will be harder (plus, the skillset needed at the sharp end of the Vuelta starts to differ ever more from that needed at the Tour).