The wider the panel the, the smaller area it has, while still preserving the diagonal (i.e. 14" 16:9 has smaller area than 14" 16:10), allowing more panels to be cut from the same material and increasing the yields in case of material defects.

I.e. making of wider panels is cheaper, and because in advertisement there's diagonal and not area, it still allows advertising in a way that doesn't put the manufacturer or oem to disadvantage.