That's not Michelin requirement but the tyre size is strictly set in supplementary regulations of the event. Usually they allow narrow tyres for small cars but not for 4WD. The reason is partially in Monte Carlo being declared as asphalt event, i.e. with asphalt brakes and asphalt suspension package. For using small snow wheels You need to change a lot of other things on the car (for example asphalt brake discs can't fit into them).

In times of IRC it was still allowed to cut the wide tyres extensively and in those times in case of big snow they for example completely removed the middle stripe of rubber blocks from the tyre and every second block from side stripes. The tyres looked almost like for tractor - like on these pictures: