There are a few reasons why this could have happened. Mostly what the big deal here is that because fuel tanks are contained within the wings there is always a worry that when a plane skids off of a runway that the wing or wings could come in contact with a runway marker or pole and cut through the wing, which could cause an explosion. I am sure that the NTSB will investigate this event (as they call these incidents) and then we will learn why the plane skidded off of the runway.
I have made several landings during snow in Chicago and Seattle. Chicago's O'Hare Airport does a fantastic job of keeping the runways cleared of ice and snow. Seattle (Sea-Tac) is OK, but I always depended on the opinions of other pilots that would communicate their assessment of the runways over the radio before landing. Their feelings of the runways conditions meant more to me than the ATC's announcement.
This is something that we train for in the simulator from time to time. It is not a high priority item because most airports do keep their runways clear and dry. When landing, most pilots will slow the speed of the aircraft down to very minimal before touchdown and place the accelerators in the idle position. Upon touchdown, pilots will engage the thrust reversers and hold onto the tiller to keep the plane as straight as possible. If the plane begins to slide, we do the same as you would do if you were skidding while driving your car, using the nose wheel to come straight again and lightly using the brakes. Landing speed is generally around 180 kts. +/- 10-30 kts. depending on weight and other variables.
