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The phrase common sense originates from the Latin sensus communis and the Ancient Greek koine aisthesis (κοινὴ αἴσθησις). In classical philosophy, particularly in Aristotle's writings, "common sense" referred to an internal faculty that unified the information from the five physical senses, allowing an organism to perceive a coherent reality. This faculty was considered distinct from rational thought but essential for basic perception and judgment.

That's why. This is the original use. It mutated over time, however:

Over time, particularly by the 16th century, the meaning shifted toward "ordinary understanding" or "basic intelligence," the kind of practical judgment expected of any rational person.

And that then further mutated into the current, somewhat contrafactual meaning:

The phrase came to signify both the shared, basic intellectual capacities of ordinary people and, sometimes, the conventional wisdom or prejudices of the majority.