Typically wider tires aren't snow rated. The idea of snow tires is to get traction on the road surface. Wider tires tend to float on the snow, rather than penetrate the snow to get to a firmer surface. Wide tires are GREAT for weight dispersion in the frozen tundras, but in civilized areas with designated roads and pathways, it's preferred to bite THROUGH the snow, and not float on top of it.
Another reason why BFG's aren't all snow rated is their tread design is very blocky. While stable in most driving conditions, the size of the blocky tread can prohibit flexibility based on size. Think of it this way. You have two pencil erasers, one on a standard pencil, and the other the size of a box of matches laying on it's largest side. Which one of those erasers would be easier to flex horizontally while maintaining constant contact with the surface? The smaller one. Since the BFG AT is a well known 5 row blocked pattern on ALL sizes, they have to enlarge the size of the blocks to keep the pattern as well as keep the tire stability as the tires get wider. Wider tires, bigger blocks for firmness, less horizontal block flexibility.
The key to a goo snow tire no matter the size is "SIPES" small cuts in the blocky patterns to allow horizontal flexibility.