Various reasons for semi trucks not putting on winter tires:
1. Weight per tire specially when loaded. The Gross Weight for an 18 wheeler is 80,000 lbs, 12,000 lbs for the steer axle and 34,000 lbs for the drive and trailer axles. Assuming the vehicle is at it's Gross Maximum, then each of the 2 front tires is bearing 6000 lbs of weight i.e. a little more than a Chevrolet Suburban, on one tire. So it is going to get better traction. Even the drive and trailer axle tires are carrying 4000 + lbs weight. Having said that, this alone will not save the vehicle if road conditions are icy.
2. The typical long distance trucker in Canada could be going from -15C in Toronto to +20C in Florida in 48 hours. When cost is an issue in an industry with thin margins why would you have fast wearing winter tires when you are driving thousands of kilometers in temperatures which will just wear out the winter tire.
3. And of course cost.