Why is it called "common" sense if it isn't common?
What are you talking about? Common sense IS common. Ex: It's common sense not to hit on a widow at her husband's funeral.
Maybe it's meant to be used as an Oxymoron.
Because we wish common sense was more common?
It is common, we just tend to focus on the stupid things people do. It's pretty boring to point out all the normal things people do, or events that went smoothly.