One of the biggest problems when driving in snow, as Culzean says, is the amount of tyre in contact with the surface. This limiting factor is compounded by the people that clearly do not have a clue. You see them time after time whether it's 4WD, rear wheel drive or even front wheel drive, sitting on a patch of snow, revving their engines with wheels spinning like the clappers, polishing what starts as snow and quickly turns to a nice patch of compacted snow/ ice. Not only do they get nowhere fast but make it dangerous for those that try and drive properly.
There are times of course when there is no way that a vehicle can move nor even pedestrians when snow has melted and then turned to ice. Or rain has frozen. You see plenty of examples on the TV in the 'comedy' clips.
I am mindful of an accident and a colleague attending , despite his training went out with the Land Rover and stopped on a hill which was sheet ice. The Landrover did have a very efficient handbrake but this did not stop the vehicle sliding down the hill he had stopped on. Hadn't even turned the front wheels into the gutter. BUT at the end of the day, given such severe conditions even winter tyres won't cope.
Once again it comes back to the amount of tread on a tyre, which in very heavy rain is very important as a skinny tyre won't clear the surface water with aquaplaning the outcome whether or not it's summer or winter tyres.