Different technology. An aircraft requires forward movement to create lift. The icing on the leading edge of an aircraft wing increases drag, adds weight, decreases thrust, and as a result lift decreases. Air moving over the top of a wing moves faster than under it creating low pressure above and high pressure below which generates lift. If the wing shape changes due to ice and the air over the top and bottom move by the wing at the same speed then it falls out of the sky, regardless of the aircraft's speed. There is also the 350-450mph wind causing windchill at altitude that contributes to icing. Rotary aircraft like helicopters are far less susceptible to icing on their lift generation surfaces, which for them are blades not wings.