* Costuming: The show's wardrobe department likely sources clothing from various brands and manufacturers to achieve a specific look for each character.
* Practicality: The shirts are likely chosen for durability and comfort, as Selleck's character, Frank Reagan, is often seen in physically demanding situations.
* Style: The shirts are designed to fit the character's image of a traditional, authoritative, and rugged police commissioner.
It's worth noting that, while specific brands might not be confirmed, you can find many similar flannel shirts with the same classic styling and color palettes through various retailers.