because the yoke in those cars (and the early 80s cars and trucks) is cast aluminum. When you turn the key, it turns a rack that extends from the yoke and that actuates the ignition/run. That yoke or the rack breaks. Then, you have to take the column completely apart - including the tilt gimbal - to extract the part and replace. Try doing that in a pickup outside in the street in 20F weather sometime. I've repaired 3 cars with that problem, don't ever want to do another.
1967 Falcon 4 door 351C-4V
1970 Mustang 351C-2V
http://raceabilene.com/kelly/hotrod
Owner built, owner abused.