The area would be an even bigger number. Wonder if marketers determined the concept of area was too complicated compared to the concept of a linear diagonal measurement.

Or a 2 digit number is easier to remember and digest than 3 or 4 digits, and 1 digit is too small, such as measuring in squared feet or meters.

Historically the diagonal was a natural metric because it corresponds to the diameter of the early round CRTs (e.g. https://commons.wikimedia.org/wiki/Category:7JP4)