One of the reasons to put tracks on a vehicle is to reduce the pressure it exerts on the ground. A bulldozer might weigh 15x as much as light truck, but if it has 75x the contact area, it would have (much!) lower ground pressure, and would be dramatically less prone to sinking into snow or mud or what-have-you.

Some sanity checks:

1993 Toyota 4Runner / Hilux Surf: 25 psi (https://en.wikipedia.org/wiki/Ground_pressure)

Caterpillar D6R LGP Bulldozer: 5 psi (http://www.ritchiespecs.com/specification?type=Con&category=...)

Some tracked vehicles like bulldozers are available in extra-wide track versions to reduce their ground pressure even further. It wouldn't be surprising if most of the snow fleet is so equipped.