Winter tires have compounds that are better suited for winter driving by staying softer at lower temperatures(duh!), the thread pattern is also quite different. While summer tires tend to have as much rubber as possible making contact with the road, winter tires tend to have a thread pattern to evacuate snow and go through it as much as possible (Lots of ridges and small grooves on winter tires for example).
If summer tires were good on snow, they would not have invented winter tires
