The image below shows the two main ways of climbing Mont Blanc in summer. The straight line on the first part of the blue Cosmiques route is the Aiguille du Midi cable car which takes one straight to 3842m one might think therefore that this was the easiest way of getting to the top of Mont Blanc. Look more closely however and you will see how much steeper the ground on the blue route is compared to the red Gouter route, and how much more climbing and descending is involved. Despite starting higher, the effort required to get up and down the blue route is in fact significantly more than for the red route.
When it’s cloudy or windy rescue helicopters can’t fly and teams suddenly have to become very self-reliant. If someone gets altitude sickness mid way along the blue route they could have major problems climbing back up to get out, which means we require higher fitness levels from clients (to give us bigger safety margins) in order to be comfortable attempting the blue route.
Finally, the steep faces on the Cosmiques route are much more prone to avalanche than the relatively gentle slopes and ridges of the Gouter route.
Hut bookings on Mont Blanc have to be made months in advance to be sure to get in and cannot easily be changed last minute when we know what conditions will be like for your particular week. We have to make a plan that will work in the majority of cases, and if we booked at the Cosmiques there would be a chance of getting avalanche conditions on the blue route when the red route was still practicable, and being unable to change bookings.